fix: await bundle closing

[benmccann]

Description

Vite does not await when calling bundle.close which causes code expected to run after the closeBundle plugin hook to run before it instead

Closes vite-pwa/sveltekit#21

Additional context

A recent refactoring in SvelteKit caused this issue to be hit extremely frequently

Vite does do an await in the other location where it calls bundle.close:

vite/packages/vite/src/node/plugins/worker.ts

Line 132 in 5444781

await bundle.close()

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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