Automatically report on discrepancies with caniuse

[ddbeck]

Right now, I'm spending quite a bit of time comparing caniuse version numbers with computed statuses derived from BCD. This is slow and error prone. Fixing errors in BCD and raising issues on caniuse can slow or block otherwise straightforward feature entries.

We ought to have a script (or an addition to caniuse.ts) that compares computed statuses with caniuse data.

It should emit errors when:

• A computed status's absolute browser support is in conflict with caniuse (e.g., if caniuse says Safari supports a feature, but web-features headline status says it's unsupported, then that's an error)
• A computed status's keystone version number (i.e., the version giving the baseline low date) differs from caniuse's version number for the same browser by more than one calendar year (e.g., if caniuse says a feature last shipped in Edge 79, but web-features says Edge 12, then that's an error)

It should emit warnings when:

• A computed status version number differs from caniuse's version number for the same browser by more than one calendar year, for any release before 2020
• A computed status version number differs from caniuse's version number for the same browser by one release, for any release after 2019

[ddbeck]

cc: @Elchi3, @jamesnw, @autonome, @atopal: I wrote this up after our call today discussing what was slowing us down from adding features, IDing a potential alternative to only doing caniuse discrepancy resolution in reviews (plus a long-term maintenance tool).

In the meantime, we might want some kind of manually-set editorial field where we could keep prose notes about likely discrepancies.