tagfilter: deprecate.

[kivikakk]

See #829.

[codspeed-hq]

Merging this PR will not alter performance

✅ 1 untouched benchmark

---

_{Comparing kiv/tagfilter-deprecate (6fd1258) with main (c5a3b45)}

[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./bench.sh ./comrak-c3f76e9	115.1 ± 1.2	112.8	118.1	1.96 ± 0.04
./bench.sh ./comrak-main	115.1 ± 1.2	112.7	117.9	1.96 ± 0.04
./bench.sh ./pulldown-cmark	58.6 ± 0.9	57.2	62.3	1.00
./bench.sh ./cmark-gfm	89.9 ± 7.9	82.3	104.5	1.53 ± 0.14
./bench.sh ./markdown-it	268.3 ± 8.3	256.4	295.4	4.58 ± 0.16

Run on Sun Aug 30 10:54:38 UTC 2026