Introduce compute_from to pin a feature's status to BCD key(s)

[foolip]

No description provided.

[foolip]

There are things I'd like to improve here still, but this shows the promise of it.

[ddbeck]

For @captainbrosset and anyone else following along:

The idea here is to let us pick a subset of a feature's compat keys to compute the Baseline status from. Applications for this would be, for example, to:

• Align a feature to caniuse's reported availability
• Capture a feature's interoperable date despite minor additions to that feature
• Ignore specific faulty/weird/complex/missing keys in BCD while still maintaining a connection to the underlying data

In general, this would let us associate more keys with features without minting¹ new features or moving a feature's Baseline low date upon later additions. Accepting this PR would depend on accepting #1204, too, so that consumers could show the most-correct status for a given slice of a feature.

Discussion relating to this idea comes mostly from #1160 and (to a lesser extent) #866 and a few (regrettably unrecorded) conversations between Philip and myself.

¹ but it also doesn't foreclose on later minting features, by cleaving keys off an existing feature

(@foolip please correct me if I've mischaracterized anything here)

[foolip]

@ddbeck that is well summarized, thank you. The main use case for this is to "lock" a feature's support status to its entrypoint(s), so that a feature that when a feature evolves the Baseline date doesn't keep moving forward.

[foolip]

@ddbeck I'd like to document how to use compat_features but I'm not sure where. See #1159.

[ddbeck]

Happy with this, but I have one straggling thought and a request to preserve comments. Thank you!