PR #17461 added the wait function to reactivity/async.js and modified the compiler to generate $.wait() calls, but forgot to export it from index.js.

This causes a runtime error when using multiple await expressions inside $derived() with experimental.async: true:

TypeError: $.wait is not a function. (In '$.wait([$$promises[20], $$promises[20]])', '$.wait' is undefined)

The $.wait function is called by the compiler when there are multiple async blockers that need to be waited on together.

Fixes #17529

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Tests and linting

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Added test async-derived-multiple-blockers which uses multiple async deriveds together, triggering the $.wait call that was previously missing.