Test for breaking changes in Gutenberg plugin _before_ they're released in Core

[justlevine]

While test-plugin-nightly.yml is an honest attempt, since most changes happen in the Gutenberg plugin and don't always get merged until late in the release cycle, we should be testing against that as well.

Flagging incompatibilities before they break the ecosystem gives us time to address them in an optimal way (vs an urgent patch/workaround) or even provide feedback upstream on changes.

With core major releases going on hiatus for a year, there will be even more gutenberg-plugin locked changes, and likely an increase of people using it instead of waiting 12 months for things to get merged into core.

[colinmurphy]

Thanks @justlevine

Definitely agree with you here and thanks for raising this issue. I agree with trying to be proactive rather than reactive to any breaking changes.