try to alert (via webhook) when Mergify is stuck

Add a label "waiting too long" if a PR has "merge delay passed"
but is still open without any activity. Somewhat hacky, since
GitHub's API doesn't provide any real hooks for it so I'm guessing
at ways to try to catch it. Can only be tested by merging, since
Mergify won't apply it unless it's on `master`.

.github/mergify.yml

@@ -62,6 +62,31 @@ pull_request_rules:
       - check-success=Bootstrap post job
       - 'check-success=docs/readthedocs.org:cabal'
 
+  # label when Mergify didn't trigger a merge automatically
+  - actions:
+      label:
+        add:
+          - waiting too long
+    name: Mergify hasn't merged a PR yet
+    conditions:
+      - base=master
+      - -draft
+      - -closed
+      - -merged
+      - '#approved-reviews-by>=2'
+      - '#changes-requested-reviews-by=0'
+      - updated-at<4 days ago
+      - label=merge delay passed
+      # oy
+      # lifted these from branch protection imports
+      - check-success=fourmolu
+      - check-success=hlint
+      - check-success=Meta checks
+      - check-success=Doctest Cabal
+      - check-success=Validate post job
+      - check-success=Bootstrap post job
+      - 'check-success=docs/readthedocs.org:cabal'
+
   # rebase+merge strategy
   - actions:
       queue: