cm::escape_link_destination: add function to escape URL for use as link destination.

[kivikakk]

Add a method that takes a URL and returns something suitable for use directly as the link destination in a CommonMark document.

[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./bench.sh ./comrak-9dd0d13	320.7 ± 3.1	313.8	326.6	3.00 ± 0.04
./bench.sh ./comrak-main	319.7 ± 2.4	316.6	326.2	2.99 ± 0.04
./bench.sh ./pulldown-cmark	106.9 ± 1.1	105.2	109.6	1.00
./bench.sh ./cmark-gfm	116.5 ± 1.3	113.9	118.6	1.09 ± 0.02
./bench.sh ./markdown-it	482.6 ± 4.6	476.2	494.3	4.52 ± 0.07

Run on Wed Sep 24 06:36:05 UTC 2025