Add Manifest Incubations proposal

[foolip]

This document contains some features that are implemented and shipped:
https://developer.mozilla.org/en-US/docs/Web/API/BeforeInstallPromptEvent#browser_compatibility

[foolip]

cc @sideshowbarker

[tidoust]

Fine with adding the spec if it's needed in MDN. We had it in the monitor list for now with a "Chromium specific" comment:

browser-specs/src/data/monitor.json

Line 215 in 606a7dc

"WICG/manifest-incubations": {

Could you drop that entry from the monitor list?

[ddbeck]

There's some interest in citing this document in mdn/browser-compat-data#13565, if that influences anything here.

[ddbeck]

(Actually, I spoke too soon. That's still in an origin trial, so it won't merge for BCD. Though there may be other things more widely available which would qualify, I don't know of any live PRs yet.)

[Elchi3]

mdn/browser-compat-data#13829 wants to use this, too.

[tidoust]

Right, the only thing that was blocking the PR was that the entry had to be dropped from the monitor list ;)

Now done. I'll merge and release a new version of browser-specs when possible (CSS drafts server is down for the time being)