Track implementation status for features

It would be good to have an easy way to show if a feature exists or not. For example, a status field that could be something like:

- No proposal
- Proposal
- Proposal Accepted
- Experimental Support (Canary Implementation/Behind flag)
- Implemented in some browsers (one or more major browsers)
- Implemented in most browsers (all major browsers except one?)
- Implemented in all browsers (all major browsers)

Thoughts?

No files, tests, or entry points are named. First inspect how feature records are represented and displayed, then clarify and agree on the status taxonomy and where it should be maintained; done means the agreed statuses are tracked and visible consistently.

Written by the indexing model from the issue text.