Fix current component not being cleared after component update

[rdb]

Fixes #4899
Fixes #4259

This PR clears the current component after the update. This has two effects:

• The lifecycle functions now raise an error when used outside of Svelte code (eg. in a setTimeout handler). Previously, they operated on the last-updated component, which may not even be in a stable state.
• The lifecycle functions now raise an error when used inside onMount and afterUpdate. If this is undesirable, we would need to explicitly call set_current_component in flush() again when calling the render callbacks.

This is essentially the same change as #4864, but that PR's unit test did not work correctly and left Svelte in an invalid state, causing many other unit tests to fail.

If any other unit tests or changes are needed, let me know and I'd be happy to work on them.

Closes #5077 (which is a subset of this fix that is more focused to only address #4899).

[Conduitry]

@tanhauhau Does this look reasonable to you? What was your manipulation of require.cache in the test in #4864 intended to do?

[tanhauhau]

Oh. Exactly what @rdb explained in #4864 (comment)

Was trying to clear the require cache of svelte/internal so the subsequent test can require a fresh state of svelte/internal

It works on mm machine though, hvnt checked why it failed in CI

[rdb]

Is there some way I could help to move this PR further along?

[antony]

@rdb we just need a bit of time to understand the full ramifications of it before we merge. Apologies for any delay.