perf: match only unpaired surrogates when escaping HTML

[Nic-Polumeyv]

Resolves the TODO in escape.js from #4024, which planned to simplify the surrogate pattern with lookbehind assertions once widely supported. Lookbehind has been safe everywhere relevant since Safari 16.4, but \p{Surrogate} with the u flag is simpler still and supported even longer (ES2018). Under the u flag a valid surrogate pair forms a single astral code point, so \p{Surrogate} matches only unpaired surrogates and the pattern's other two branches disappear, including the branch that existed only to match valid pairs so the replace callback could return them unchanged.

That branch is also why this is a perf change rather than a cleanup. Every valid pair in rendered content currently invokes the replace callback just to pass through. With the new pattern pairs never match, so escape_html measures about 2x faster on emoji-heavy content in a quick microbench, with plain ASCII at parity.

Output is unchanged. I diffed the old and new implementations over 200k fuzzed strings built from surrogate halves, dict characters and astral pairs, plus curated edge cases, in both modes, all byte-identical. escape.spec.js passes as-is.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 29ac8ca:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/29ac8cae9564fac5960dad2b62d72e03fd3cf843

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16407

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Latest commit: 29ac8ca

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[Rich-Harris]

love it!