Crawl prerendered pages without regex hackery

[Rich-Harris]

This is an alternative to #2923. It prevents the sort of bugs that can arise when you're extracting attributes with regex...

<img data-something=" src=" crossorigin src="dog>cat.png"> crazy link </img>

...without the overhead of fully parsing the HTML. parse5 is great, but it's much too large to add on a whim, and does far more than we need. This implementation takes a tiny fraction of the size (it adds less than 100LOC) and is roughly 3x faster (I'm sure someone suitably motivated could make it faster still), which is important in this context.

Fixes #2645 and #2742.

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[JackPriceBurns]

@Rich-Harris in packages/kit/src/core/adapt/prerender/fixtures/large-page/input.html can we also add some <source> and <img> tags with the attributes in different places to ensure that #2742 never happens again (will protect us from regressions if we ever decide to move to parse5 or something else later)