Add Assign Issue workflow (#10408)

Allow users to assign issues to themselves.

I don't know if its comments (i.e. unassigning) affects orca bot's
inactivity and stale. Probably. I didn't see a way for actions/stale
to exempt particular comments. This is set for 30 days and stale for
90 days, so if people are using this the stale timing can jump up to
120 days.

.github/workflows/assign.yml

@@ -0,0 +1,24 @@
+name: Assign Issue
+
+on:
+  schedule:
+    - cron: 0 0 * * *
+  issue_comment:
+    types: [created]
+  workflow_dispatch:
+
+jobs:
+  assign:
+    permissions:
+      issues: write
+    runs-on: ubuntu-latest
+    steps:
+      - name: Assign the user or unassign stale assignments
+        uses: takanome-dev/assign-issue-action@v2.2
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          maintainers: 'noisyfox,softfever'
+          days_until_unassign: 30
+          block_assignment: false
+          reminder_days: 7
+          max_assignments: 12