Add GHA dependency check

[cds-amal]

This PR builds on #5230 to address #5236 by adding a GitHub action to CI. I didn't create a separate job for the dependency check because it would require the expensive yarn bootstrap command and I don't want CI to take longer. Instead, I inserted the dependency-check into the matrix strategy build job to take advantage of the matrix's short-circuiting behavior. If one matrix job fails, it cancels all others. I also updated the Slack-app config and made the matrix-build job a dependency of yarncheck. yarncheck resolves in ~2.5 minutes and then the matrix job runs, followed by Slack notification.

If this is approved, I will rebase to remove all simulated errors

[haltman-at]

Hm, would it make more sense to just put the test in the yarncheck job? My understanding was that a failure of yarncheck would also cancel the others... is that not the case? That's what I've always observed...

[haltman-at]

I think we're probably better off without the explicit custom error message here; I think the existing error is clear enough and the method for including it is kind of hacky. I only put an explicit custom error message on yarncheck because otherwise it's just not clear at all with that one.

(If yarn --frozen-lockfile worked like it was supposed to, and I didn't need the custom test, then that would probably be clear and I could omit the custom error... but it doesn't, so yeah.)

[cds-amal]

Sure, I'll remove that.

[cds-amal]

Simulated an error and it looks ok.

[cds-amal]

Hm, would it make more sense to just put the test in the yarncheck job? My understanding was that a failure of yarncheck would also cancel the others... is that not the case? That's what I've always observed...

I would prefer doing these build/lint type checks in one job, but dependency-check needs yarn bootstrap to build out the entry points; I wanted to avoid adding another 12-13 minutes to the yarncheck job. I'm also unsure why the cancel behavior changed, as I remember it working as you did, but testing showed otherwise.

[haltman-at]

Makes sense!

[cds-amal]

Will rebase to remove the simulated errors