Currently, not every part of your component can be async— child fragments can't be. A fragment is the children inside of a block, such as an {#if } block. Most fragments are compiled to synchronous functions. Since template expressions are individually compiled to functions that are either async or sync depending on their values, they can be async inside synchronous fragments. However, other things, such as {@const }, can't be async, since you can't have await expressions inside synchronous functions. This fixes that. Additionally, this makes certain features, such as variable declarations in markup, much easier to implement.

Closes #16462 Supersedes #16463

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Tests and linting

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