fix: correctly handle functions when determining async blockers

[dummdidumm]

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

For review: The second commit is just moving things around

Also unrelated-but-appeared-in-the-same-issue-fix: take blockers into account when creating #await blocks

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Latest commit: a4486b2

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Playground

pnpm add https://pkg.pr.new/svelte@17137

[Rich-Harris]

outstanding