Only inject hook.js script for Firefox. Use ExecutionWorld.MAIN for Chrome

[jerelmiller]

Fixes #1162

This is a revival of #1164 with some tweaks. Now that we are running Chrome on Manifest v3, we are able to use hook.js as a content script using ExecutionWorld.MAIN. Unfortunately this is not yet supported in Firefox, so we have to rely on script injection for this to work. The script injection used in #1164 is now added back, but only when building for Firefox.

[phryneas]

This looks good to me (tested locally in Chrome and Firefox), but I'm slightly concerned: didn't you see the extension break with the current Firefox code, in Firefox?

Could you reproduce that?

[jerelmiller]

Talked in our 1:1. Posting the answer here for transparency:

I feel a bit more confident now in this change for a couple reasons:

• We now unconditionally show the devtools where before we relied on a message sent to devtools before the panel was created (Always show the devtools panel, regardless of whether we detect a client #1220)
• Message passing has been reworked for stability (Rework message passing #1226)
• The injection mechanism is now relegated to Firefox only now that Chrome is on manifest v3 (Migrate to manifest v3 for Chrome #1225)

[jerelmiller]

We are going to wait to merge this for a few days after 4.7.0 goes out to ensure we catch any bugs or new issues that arise from that release.

[jerelmiller]

Putting in draft for now to avoid accidental merge until we are ready.

[jerelmiller]

So far we've seen no reports that the latest update caused any more issues. I'll go ahead and merge this and get a release out tomorrow.