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

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