commonmark: experimental minimize

[charlottia]

Fixes #509, kinda.
Experimental-quality CommonMark output minimizer.

Doesn't encode any intelligence about the underlying format; tries removing every backslash (\\) escape in order and sees if the document parses the same, accumulating those deletions that do.

It's wasteful but better than nothing!

[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./bench.sh ./comrak-cc84aa7	316.3 ± 1.0	314.5	319.1	2.91 ± 0.03
./bench.sh ./comrak-main	318.6 ± 2.7	314.7	329.3	2.93 ± 0.03
./bench.sh ./pulldown-cmark	108.8 ± 0.9	107.4	111.2	1.00
./bench.sh ./cmark-gfm	115.7 ± 1.4	113.3	119.2	1.06 ± 0.02
./bench.sh ./markdown-it	486.8 ± 10.6	475.2	523.3	4.47 ± 0.10

Run on Tue Jan 21 09:39:02 UTC 2025

[kivikakk]

Tried here, works well: kivikakk/kivikakk@2f99e2f.