Add a regularly running action that checks for spec PRs

[captainbrosset]

Sometimes we link to open PRs in lieu of feature specs, because the PR is the closest thing we have to a spec. Later, when those PRs merge, we should revisit the features and update the links to actually point to the real spec.

Examples: #3450 #3451.

[ddbeck]

Yes, I think this would be a nice complement to something like my proposal in #3470, to be more permissive with spec URLs for some features via a distinct authoring field.