.github/workflows: add CI run to lock closed threads.

Closed PRs will be locked after 90 days of inactivity, to avoid comments
on old inactive threads and encourage people to create new ones, which
will receive the proper attention.

Inspired by hook from fish-shell:
a36dbad3b8/.github/workflows/lockthreads.yml

.github/workflows/lockthreads.yml

@@ -0,0 +1,14 @@
+name: 'Lock threads'
+
+on:
+  schedule:
+    - cron: '0 18 * * *'
+
+jobs:
+  lock:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: dessant/lock-threads@v2
+        with:
+          github-token: ${{ github.token }}
+          pr-lock-inactive-days: '90'