Main world injection: Prevent race condition

[marcustyphoon]

Description

#1514 relies on the fact that fetching a json file and performing an extension storage query will take longer than the browser fetching the main_world script file, as we need the latter to be performed before importing modules with side effects or executing code that uses injection.

This performs a back-and-forth handshake to confirm that the main_world script file has been executed before proceeding with main install.

Random aside: to improve extension load speed slightly, one can do:

    const [
      installedScripts,
      { enabledScripts = [] }
    ] = await Promise.all([
      getInstalledScripts(),
      browser.storage.local.get('enabledScripts'),
      mainWorldReady
    ]);

but I can't figure out how to make the code not look stupid.

Testing steps

• Confirm basic extension functionality.
• If desired, unrevert the test commit and confirm that basic functionality appears with a significant delay on page load.

I have no tests for the bad case this prevents, as I can't for the life of me get it to happen.

[AprilSylph]

but I can't figure out how to make the code not look stupid.

I don't think it looks that stupid, and it serves a clear purpose of "do these things in parallel instead of sequentially".
I've also seen the exact same pattern in the Redpop codebase.

[marcustyphoon]

Yeah, I mean, it's definitely the way that you do that. I just wish the inputs and outputs lined up... and that there wasn't a mismatch in the inputs and outputs in this case.

(Thought: I guess in a typescript codebase it's less important to make it visually clear in code what expressions correspond with one another, because you'll get type errors immediately if you change things in an erroneous way. Vanilla javascript imposes more discipline in that regard, which can be inconvenient but can also be good practice!)