[ENHANCEMENT] show support percentage for TC39 features via test262 result data

[michaelficarra]

Where some features show support percentages, features defined by TC39 show this message:

Show the test262 pass percentage for the feature instead. All TC39 proposals have an associated test262 feature flag so that you can collect all the tests. See https://test262.fyi/, a website that does this already.

[past]

web-platform-tests/interop#117 is relevant here. @foolip is it better to wait for an Interop-endorsed approach here or can WebDX provide test262 annotations for Web Features?

[foolip]

I previously took a look at using test262.fyi results, and @CanadaHonk kindly helped expose some data that would be useful for this is in test262-fyi/test262.fyi#51. The biggest issue is that there's no data corresponding to stable releases of browsers, and that is what we show by default on webstatus.dev. (For experimental we could use V8 results for Chrome+Edge, SpiderMonkey for Firefox, and JavaScriptCore for Safari.)

I think that Test262 results from the same browser binaries that show up on wpt.fyi would be the most useful. Something like web-platform-tests/wpt#8980 / web-platform-tests/wpt#35621 is on way to go about it that I think is promising. An alternative path would be to run set up real browsers for test262.fyi and use those results. That might in fact be easier, but I'm not as familiar with what it would take.

[foolip]

https://github.com/tc39/test262/blob/main/features.txt is the file in Test262 defining feature identifiers, which are then used in tests. That's what gets surfaced on test262.fyi, IIUC.