avm2: Work on flash.system.Capabilities

[Lord-McSweeney]

Eventually, code from #10469 can be reused to implement the rest of the flash.system.Capabilities getters.

Fixes #9180.

[iwannabethedev]

I do not understand Rust or this piece of Ruffle well enough to really give good feedback, but it looks good to me overall.

I imagine that this can be complicated to test in an automated way. Have you tested the two new properties manually?

I think someone else should review as well, but otherwise this looks good to me as is. You are welcome to close the comments that I made once you have read them.

[Lord-McSweeney]

I've tested both the properties, with --player-type, and compiling this branch and running a SWF in the browser/desktop app (which correctly reports StandAlone and PlugIn).

[iwannabethedev]

I believe that @n0samu thinks this looks good, and I myself think it looks good (with my limited insight into Rust and this part of the codebase), so I believe it is ready to be merged once rebased.