ci: Add a job to nag when changesets are missing (#2450)

Mergify will now add a comment asking if you forgot a changeset if you change TS, Go, or JS code without adding one.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.github/mergify.yml

@@ -97,3 +97,12 @@ pull_request_rules:
       label:
         remove:
           - on-merge-train
+  - name: Nag changesets
+    conditions:
+      - and:
+        - 'files~=\.(ts|go|js|mod|sum)$'
+        - '-files~=^\.changeset/(.*)\.md'
+    actions:
+      comment:
+        message: |
+          This PR changes implementation code, but doesn't include a changeset. Did you forget to add one?