chore: add gh action to lock old issues and PRs (#1888)

As per our [contributing guidelines](https://github.com/typescript-eslint/typescript-eslint/blob/master/CONTRIBUTING.md#commenting), we prefer if users raise new issues instead of commenting on old, closed ones.
This adds a github action to automatically lock issues and PRs after they've been closed for 30 days.

.github/workflows/lock.yml

@@ -0,0 +1,20 @@
+name: "Lock threads"
+
+on:
+  schedule:
+    # TODO - change to "0 0 * * *" once the backlog is processed
+    - cron: "0 * * * *"
+
+jobs:
+  lock:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: dessant/lock-threads@v2
+        with:
+          github-token: ${{ github.token }}
+          issue-lock-inactive-days: "30"
+          issue-lock-reason: "resolved"
+          issue-lock-comment: ""
+          pr-lock-inactive-days: "30"
+          pr-lock-reason: "resolved"
+          pr-lock-comment: ""