feat(actions): auto-label PRs that close issues.

[rusackas]

SUMMARY

Just a theory, but this should look at any given PR and check the description body to see if it auto-closes any issues. If so, label the thing! This should help us prioritize PRs that will also help whittle the Issues backlog.

This also looks at more triggers (including "edited") to run the action on PR description edits. The jobs in the action can be triggered by subsets of these actions (e.g. the main labeler doesn't run on PR description edits).

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

Go ahead! Edit this PR description, and look for the addition/removal of the label!

ADDITIONAL INFORMATION

• [ ] Has associated issue:
• [ ] Required feature flags:
• [ ] Changes UI
• [ ] Includes DB Migration (follow approval process in SIP-59)
  • [ ] Migration is atomic, supports rollback & is backwards-compatible
  • [ ] Confirm DB migration upgrade and downgrade tested
  • [ ] Runtime estimates and downtime expectations provided
• [ ] Introduces new feature or API
• [ ] Removes existing feature or API