chore: simpler fallback values

[Rich-Harris]

Was reminded of this by #12781. We currently make things very complicated when rewriting destructuring patterns that have default values — this...

[a = await promise] = stuff();

...becomes this...

await (async () => {
  const tmp = stuff();

  (
    $.set(a, $.proxy(await $.value_or_fallback_async(tmp[0], async () => await promise)))
  );

  return tmp;
})();

As of #12780 it becomes this instead, which is simpler...

await (async ($$value) => (
  $.set(a, $.proxy(await $.value_or_fallback_async($$value[0], async () => await promise)))
))(stuff());

...but really it should just be this:

await (async ($$value) => (
  $.set(a, $.proxy(await $.fallback($$value[0], promise)))
))(stuff());

This PR makes a few changes:

• it renames value_or_fallback to just fallback, and moves it to the shared directory since it's currently duplicated between server and client
• it removes value_or_fallback_async, which doesn't do anything
• it adds a build_fallback function instead of duplicating logic between the various places that need to create $.fallback code
• it makes lazy fallback values the exception. If we have something [answer = 42], we don't need to wrap the 42 in a thunk
• the same goes for await expressions — we don't need to wrap the fallback value in an async thunk it it's just something like await promise, we can just await the whole expression

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• 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
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• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

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

[changeset-bot]

🦋 Changeset detected

Latest commit: c38ac4e

The changes in this PR will be included in the next version bump.

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