SPEC BREAKING CHANGE: respect up to 2 characters ahead

[tats-u]

Fixes #27

Markdown parsers are now required to be able to peak 2 characters ahead to deal with the following variation sequences after a delimiter run:

• U+201{8,9,C,D} + U+FE01
• text presentation sequence starting with a CJK default emoji presentation character
• emoji presentation sequence starting with a CJK symbol/punctuation

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Micromark & remark packages were implemented by Claude Opus 4.6 via GitHub Copilot just by 6 premium requests.

[codspeed-hq]

Merging this PR will not alter performance

✅ 12 untouched benchmarks

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_{Comparing pr/next-next-char (d1d2539) with main (abea3f2)}