Closes #16462

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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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• Ideally, include a test that fails without this PR but passes with it.
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Closes #16462

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

Implemented by reusing the async_body function inside Fragment.js. Also removes the ability to reference a {@const ...} of an implicit child inside a boundary pending/failed snippet:

• existing duplication of consts can have unintended side effects, e.g. async consts would unexpectedly called multiple times
• what if a const is the reason for the failure of a boundary, but is then referenced in the failed snippet?
• what if an async const is referenced in a pending snippet? deadlock
• inconsistent with how it behaves for components where this already does not work

Implemented via / only taking effect with the experimental flag so the behavior change only applies there as this is a breaking change strictly speaking. Also added a compiler error for this.

closes #16462

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint