write out %25 in hrefs where not part of a percent-encode sequence.

[kivikakk]

This doesn't change how such URIs are interpreted by browsers: if a % isn't followed by two hexdigits, it's treated as the character %. If it is, it's treated as a percent-encoded sequence.

This PR makes Comrak's output a little more regular: we explicitly output %25 where it isn't part of a percent-encode sequence. There is no effect on how any output is actually interpreted; before something like %%20 would get written out as %%20 and interpreted as the character % followed by a space; now it's written out as %25%20, which encodes the character % followed by a space.

[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./bench.sh ./comrak-8a3590c	105.2 ± 1.4	103.2	109.4	1.75 ± 0.04
./bench.sh ./comrak-main	104.5 ± 1.4	102.3	108.9	1.74 ± 0.04
./bench.sh ./pulldown-cmark	60.1 ± 1.2	58.3	63.2	1.00
./bench.sh ./cmark-gfm	94.1 ± 6.9	84.6	106.9	1.57 ± 0.12
./bench.sh ./markdown-it	268.4 ± 4.2	262.7	280.3	4.47 ± 0.11

Run on Tue Nov 11 00:42:02 UTC 2025