dev: introduce Reviewer lottery

.github/reviewer-lottery.yml

+groups:
+  - name: devs
+    reviewers: 2
+    internal_reviewers: 1
+    usernames:
+      - syuilo
+      - acid-chicken
+      - EbiseLutica
+      - rinsuki
+      - tamaina

.github/workflows/reviewer_lottery.yml

+name: "Reviewer lottery"
+on:
+  pull_request_target:
+    types: [opened, ready_for_review, reopened]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - uses: uesteibar/reviewer-lottery@v2
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}