fix: correctly handle functions when determining async blockers (#17137)

* fix: correctly handle functions when determining async blockers

We didn't properly handle functions (function declarations/expressions/arrow functions) when calculating what is a blocker. More specifically
- we did defer assignment of variable declarations even for arrow functions and function expressions, which is unnecessary and causes bugs when they're then referenced eagerly further below
- we did not compute blockers for functions. They could reference blockers themselves, as such other code referencing them should wait on the related blockers

Fixes #17129

* put into its own function

* fix: take blockers into account when creating `#await` blocks

The unrelated-but-in-the-same-issue-referenced-bug

* oops

* fix

* minimize compiled output changes

* no idea why editor showed these as unused

Author: dummdidumm

.changeset/gentle-showers-speak.md

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+fix: correctly handle functions when determining async blockers

.changeset/soft-radios-make.md

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+'svelte': patch
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+fix: take blockers into account when creating `#await` blocks