Add holding features for unspecified platform features

[ddbeck]

This PR contains features that hold compat keys for unspecified platform features. See #1497 (comment) for candidate keys. These are features of last resort for keys that cannot be assigned to a "real" feature (discouraged or otherwise), for lack of a specification.

If you have privileges on this repo, then you're welcome to push additional commits to add keys as you review them. But before you do so, please take a few minutes to confirm that:

• The key lacks a spec_url value in BCD.

• The key has standard_track value set to false in BCD.

• The feature was not formerly part of a spec or explicitly excluded from a specification (i.e., the specification process intentionally favored a standardized alternative). Such keys might be candidate for actual features (Add feature for discouraged import assertions #2565 for an example).

  Good places to find this information include:

  • The BCD key's Git blame view and BCD repo PRs
  • GitHub repos and orgs, such as w3c/csswg-drafts, WHATWG, and TC39.
  • Browser bug trackers (best for keys with single-implementations)