Manifest V3-compatible injection demo

[marcustyphoon]

Description

This is a functional MV3 port (without the change to manifest.json) that works in both Chrome and Firefox without per-browser modification, i.e. slightly updated #921. There are a number of methods to do our main world script injection; this one uses bubbled custom events and moves the imported function bodies into their own files, using a cached dynamic import to load them for performance (this should be compared against simply setting a script element's src property, for example).

I had to append the target element as the last argument to the injected function unconditionally; this means injected functions can't have optional arguments. We could do other stuff: make it the first argument, change the signature to take an object of named fields, whatever.

Testing steps

to write

[marcustyphoon]

• file names and locations are arbitrary/little thought was put into them
• injected function bodies in their current positions are block-commented (would be removed if merging this; this just makes for easier comparison/lower risk of merge conflicts)