Add `m`/`M` multiline substitute flag

Bug

The m / M substitute flag (multiline mode — make ^ and $ match at embedded newlines inside the pattern space) is not implemented.

Reproduction

$ printf "foo\nbar\n" | /usr/bin/sed 'N;s/^./X/gm'
Xoo
Xar

$ printf "foo\nbar\n" | ./target/release/sed 'N;s/^./X/gm'
sed: <script argument 1>:1:11: error: invalid substitute flag: 'm'

Without the m flag, ^ only matches the start of the pattern space, so only the first line gets X:

$ printf "foo\nbar\n" | /usr/bin/sed 'N;s/^./X/g'
Xoo
bar

What it should do

GNU sed manual:

M m This is a GNU extension. If specified, it causes ^ and $ to match respectively (in addition to the normal behavior) the empty string after a newline, and the empty string before a newline.

In the underlying regex engine (fancy-regex), this corresponds to turning on the (?m) (multiline) inline flag for the compiled regex.

Suspected place to add it

src/sed/compiler.rs:854 — compile_subst_flags. Around the existing 'i' | 'I' branch (line 891), add:

'm' | 'M' => {
    subst.multiline = true;     // new field on Subst
    line.advance();
}

Then in src/sed/fast_regex.rs (or wherever the regex is compiled from the substitution command), prepend (?m) to the pattern when subst.multiline is true — analogous to how subst.ignore_case adds (?i).

Rejected under --posix (handled by issue #11 once that lands).

Affected GNU testsuite tests

subst-options, posix-mode-s.