feat: Support returning values from scripts executed with the scripting API

[aklinker1]

This closes #622.

Adds support for returning values from both content scripts (likely with registration: 'runtime', but not necessarily) and unlisted scripts.

Just return a value from the script's main function and the background will get the value, even if it's an async function.

// Sync function's work
export default defineUnlistedScript(() => {
  // ...
  return "return value"
});

// Async functions also work
export default defineUnlistedScript(async () => {
  // ...
  return "return value"
});

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[codecov]

Codecov Report

Attention: Patch coverage is 55.00000% with 27 lines in your changes are missing coverage. Please review.

Project coverage is 86.49%. Comparing base (21106ce) to head (d91277f).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...irtual/content-script-isolated-world-entrypoint.ts	0.00%	9 Missing ⚠️
...rc/virtual/content-script-main-world-entrypoint.ts	0.00%	9 Missing ⚠️
...ages/wxt/src/virtual/unlisted-script-entrypoint.ts	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #624      +/-   ##
==========================================
- Coverage   86.66%   86.49%   -0.17%     
==========================================
  Files         112      112              
  Lines        9061     9111      +50     
  Branches      908      910       +2     
==========================================
+ Hits         7853     7881      +28     
- Misses       1194     1216      +22     
  Partials       14       14              

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[aklinker1]

Revert before merging

[aklinker1]

Waiting for validation from discord to make sure this works for the person who reported it...

https://discord.com/channels/1212416027611365476/1229813661821046794/1229813661821046794

[aklinker1]

Gonna go ahead and merge this, original user hasn't responded for a week or so, and there's already too many PRs in draft right now...

[aklinker1]

Released in v0.18.0