completion support coverage

in the long run, i'd like to have more external and custom completions inside both gm and sugar :+1:

in gm

• custom completion on all the exported commands, e.g. host + user + project completion, possibly by pulling GitHub (? :open_mouth:)

in sugar

external completions

• the gh CLI command of GitHub in sugar completions gh: based on gh version 2.28.0 (2023-04-25)?
• the usual git command
  • draw the full list of commands and options
  • write them all, even empty, for completeness
  • write custom completion for them
  • split the externals into modules, e.g. core, cherry, remote, worktree, ..., to allow importing only usefull commands more easily

custom completions

• sugar git
• sugar gist (removed)
• sugar gh
• sugar dotfiles (removed)