feat(schemas/browsers)!: merge accepts_{flags,webextension} fields into accepts array

[caugner]

Summary

Merges the accepts_flags and accepts_webextensions fields in the browser schema into a single accepts array that accepts the enum values flags and webextensions.

Important

This is a breaking change, as we expose these fields in the built data.json.

Test results and supporting details

Prepares for #25804, which adds webdriver-bidi.

Related issues

[queengooborg]

Considering that we're adding more flags for featuresets browsers allow, this makes sense.

Small nit: perhaps it would make more sense to make this a dictionary instead, so that we can add version numbers for when a featureset was introduced?

[caugner]

Small nit: perhaps it would make more sense to make this a dictionary instead, so that we can add version numbers for when a featureset was introduced?

@queengooborg I'm open to discussing the use case behind making this a dictionary, but I would prefer to start with this basic approach, to avoid scope creep.