cm::escape_inline: add function to escape input for inline text.

[kivikakk]

Add a method so we can escape input text, suitable for inclusion in CommonMark source where inline text processing is occurring.

[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./bench.sh ./comrak-5b4662f	312.7 ± 1.6	311.1	318.0	2.99 ± 0.02
./bench.sh ./comrak-main	312.6 ± 0.7	311.6	313.8	2.99 ± 0.01
./bench.sh ./pulldown-cmark	104.7 ± 0.4	103.7	105.3	1.00
./bench.sh ./cmark-gfm	114.1 ± 1.0	112.5	116.3	1.09 ± 0.01
./bench.sh ./markdown-it	468.1 ± 3.5	463.6	481.6	4.47 ± 0.04

Run on Wed Sep 24 05:41:36 UTC 2025