Terminal mode buffers do not implement Xterm control sequences to change cursor shape correctly

使用 mvim --clean 重现该问题，然后在终端缓冲区中将列出的 CSI Ps SP q 序列的行为与 gvim 进行比较。跟踪 MacVim 如何处理终端光标样式序列，重点关注为什么值 3–6 会变得不可见，而 1–2 可以正常工作。完成标准是闪烁和稳定的下划线及竖条样式能够正确渲染，且不会导致块样式回归。

Describe the bug
According to the VIM documentation in terminal.txt under terminal-cursor-style:

By default the cursor in the terminal window uses a not blinking block. The
normal xterm escape sequences can be used to change the blinking state and the
shape. Once focus leaves the terminal window Vim will restore the original
cursor.

These control sequences are defined as follows (source: XTerm Control Sequences ):

CSI Ps SP q
          Set cursor style (DECSCUSR), VT520.
            Ps = 0  ⇒  blinking block.
            Ps = 1  ⇒  blinking block (default).
            Ps = 2  ⇒  steady block.
            Ps = 3  ⇒  blinking underline.
            Ps = 4  ⇒  steady underline.
            Ps = 5  ⇒  blinking bar, xterm.
            Ps = 6  ⇒  steady bar, xterm.

However, trying these in a terminal inside the GUI of MacVim does not work.

To Reproduce
Detailed steps to reproduce the behavior:

1. Run mvim --clean
2. Type :terminal to create a new terminal emulator buffer
3. Type any of the following commands and press enter:
   • printf '\e[1 q' should turn the cursor into a blinking block, which works;
   • printf '\e[2 q' should turn the cursor into a steady block, which works;
   • printf '\e[3 q' should turn the cursor into a blinking underline, instead the cursor turns invisible;
   • printf '\e[4 q' should turn the cursor into a steady underline, instead the cursor turns invisible;
   • printf '\e[5 q' should turn the cursor into a blinking bar, instead the cursor turns invisible;
   • printf '\e[6 q' should turn the cursor into a steady bar, instead the cursor turns invisible.

Expected behavior
The same steps executed in gvim on Kubuntu 18.04 all work as expected, changing the cursor shapes as prescribed by the definition of the control sequences cited above.

Environment (please complete the following information):

• Vim version

  :version
  VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Sep 24 2021 14:13:28)
  macOS version - x86_64
  Included patches: 1-3455
  Compiled by GitHub Actions
  Huge version with MacVim GUI.  Features included (+) or not (-):
  +acl               +file_in_path      -mouse_sysmouse    +tag_binary
  +arabic            +find_in_path      +mouse_urxvt       -tag_old_static
  +autocmd           +float             +mouse_xterm       -tag_any_white
  +autochdir         +folding           +multi_byte        -tcl
  -autoservername    -footer            +multi_lang        +termguicolors
  +balloon_eval      +fork()            -mzscheme          +terminal
  +balloon_eval_term +fullscreen        +netbeans_intg     +terminfo
  +browse            +gettext           +num64             +termresponse
  ++builtin_terms    -hangul_input      +odbeditor         +textobjects
  +byte_offset       +iconv             +packages          +textprop
  +channel           +insert_expand     +path_extra        +timers
  +cindent           +ipv6              +perl/dyn          +title
  +clientserver      +job               +persistent_undo   +toolbar
  +clipboard         +jumplist          +popupwin          +transparency
  +cmdline_compl     +keymap            +postscript        +user_commands
  +cmdline_hist      +lambda            +printer           +vartabs
  +cmdline_info      +langmap           +profile           +vertsplit
  +comments          +libcall           +python/dyn        +virtualedit
  +conceal           +linebreak         +python3/dyn       +visual
  +cryptv            +lispindent        +quickfix          +visualextra
  +cscope            +listcmds          +reltime           +viminfo
  +cursorbind        +localmap          +rightleft         +vreplace
  +cursorshape       +lua/dyn           +ruby/dyn          +wildignore
  +dialog_con_gui    +menu              +scrollbind        +wildmenu
  +diff              +mksession         +signs             +windows
  +digraphs          +modify_fname      +smartindent       +writebackup
  +dnd               +mouse             -sodium            -X11
  -ebcdic            +mouseshape        -sound             -xfontset
  +emacs_tags        +mouse_dec         +spell             +xim
  +eval              -mouse_gpm         +startuptime       -xpm
  +ex_extra          -mouse_jsbterm     +statusline        -xsmp
  +extra_search      +mouse_netterm     -sun_workshop      -xterm_clipboard
  -farsi             +mouse_sgr         +syntax            -xterm_save
     system vimrc file: "$VIM/vimrc"
       user vimrc file: "$HOME/.vimrc"
   2nd user vimrc file: "~/.vim/vimrc"
        user exrc file: "$HOME/.exrc"
    system gvimrc file: "$VIM/gvimrc"
      user gvimrc file: "$HOME/.gvimrc"
  2nd user gvimrc file: "~/.vim/gvimrc"
         defaults file: "$VIMRUNTIME/defaults.vim"
      system menu file: "$VIMRUNTIME/menu.vim"
    fall-back for $VIM: "/Applications/MacVim.app/Contents/Resources/vim"
  Compilation: clang -c -I. -Iproto -DHAVE_CONFIG_H -DFEAT_GUI_MACVIM -Wall -Wno-u
  nknown-pragmas -pipe -DMACOS_X -DMACOS_X_DARWIN -g -O2 -arch x86_64 -arch arm64
  -D_REENTRANT -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1 -Wall -Wextra -Wshadow -Werro
  r -Wno-error=missing-field-initializers -Wno-error=deprecated-declarations -Wno-
  error=unused-function
  Linking: clang -L. -fstack-protector-strong -L/usr/local/lib -L/usr/local/opt/li
  byaml/lib -L/usr/local/opt/[email protected]/lib -L/usr/local/opt/readline/lib -L. -fs
  tack-protector-strong -L/usr/local/lib -L/usr/local/opt/libyaml/lib -L/usr/local
  /opt/[email protected]/lib -L/usr/local/opt/readline/lib -arch x86_64 -arch arm64 -L/u
  sr/local/lib -o Vim -lm -lncurses -liconv /usr/local/lib/libintl.a -framework Ap
  pKit -fstack-protector -L/System/Library/Perl/5.18/darwin-thread-multi-2level/CO
  RE
  

• OS: macOS Catalina 10.15.7

• Terminal: GUI