chore: tidy up hydration code

[Rich-Harris]

A few minor cosmetic changes:

• current_hydration_fragment -> hydrate_nodes — three syllables instead of seven, and avoids a confusing use of the word 'fragment' (which implies a DocumentFragment). In some cases, this allows Prettier to put things on a single line where previously it would have taken more
• instead of doing set_current_hydration_fragment(null) and set_current_hydration_fragment([]) as an indirect way of setting hydrating to false or true (it has no other effect), we just have a set_hydrating function. This is much easier to understand
• outside hydration.js, get_hydration_fragment and set_current_hydration_fragment were always called together. The intent of this pair of functions is to update hydrate_nodes, and as such they have been replaced with a combined update_hydrate_nodes(...) function
• moved capture_fragment_from_node into hydration.js
• simplified a few functions a bit

I believe there are some further opportunities for simplification but wanted to land this first to keep PRs manageable. No behaviour changes so I'll self-merge once green

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