Add experimental offscreen execution environment

[Mrtenz]

This adds a new execution environment that runs snaps using Chrome's offscreen API. It behaves similar to the iframe execution environment, but uses a proxy between the extension and the iframes, which runs in an offscreen document. This makes it compatible with MV3.

Closes #1088.

[codecov-commenter]

Codecov Report

Merging #1082 (92eaabd) into main (ae88c92) will increase coverage by 0.55%.
The diff coverage is 95.78%.

@@            Coverage Diff             @@
##             main    #1082      +/-   ##
==========================================
+ Coverage   94.51%   95.06%   +0.55%     
==========================================
  Files         130      129       -1     
  Lines        4299     4279      -20     
  Branches      729      718      -11     
==========================================
+ Hits         4063     4068       +5     
+ Misses        236      211      -25     

Impacted Files	Coverage Δ
...ntrollers/src/services/AbstractExecutionService.ts	90.72% <ø> (ø)
...ices/iframe/test/fixJSDOMPostMessageEventSource.ts	90.00% <ø> (ø)
packages/snaps-utils/src/iframe.ts	84.61% <84.61%> (ø)
...nvironments/src/offscreen/OffscreenSnapExecutor.ts	94.73% <94.73%> (ø)
packages/snaps-controllers/src/services/browser.ts	100.00% <100.00%> (ø)
...lers/src/services/iframe/IframeExecutionService.ts	100.00% <100.00%> (+8.33%)	⬆️
packages/snaps-controllers/src/services/index.ts	100.00% <100.00%> (ø)
...rc/services/offscreen/OffscreenExecutionService.ts	100.00% <100.00%> (ø)
...c/services/offscreen/OffscreenPostMessageStream.ts	100.00% <100.00%> (ø)
.../snaps-controllers/src/services/offscreen/index.ts	100.00% <100.00%> (ø)
... and 6 more

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[FrederikBolding]

Should we create an issue to figure out if we can remove the need for this being passed?