Add a proposal to change the execution semantics of nonisolated async functions. (#2572)

* WIP: Add a proposal for changing the execution semantics of nonisolated
async functions.

* Clarify why `@Sendable` function values can't inherit isolation.

* Add an alternative considered for using `nonisolated` to control where
an async function executes.

* Various proposal clarifications.

* Add a bit more detail to the ABI compatibility section.

* Fill out the source compatibility section.

* Fix a typo in a code example.

* Proposal revisions.

* Address feedback from the Language Steering Group.

1. Make it clear that in the end, `@execution(caller)` will rarely
   be explicit in source.
2. Simplify the function conversions section using a table, and
   clarify the rules for non-`@Sendable` function conversions.
3. Address alternative spellings for `@execution` in the alternatives
   considered section.

* Remove a confusing line of reasoning. It's not very compelling
anyway.

* Editorial changes to the proposed solution.

* Clarify that the import-as-async change is not gated behind the
upcoming feature.

* Assign SE-0461

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Co-authored-by: Xiaodi Wu <[email protected]>

Author: hborla