Add dependabot and CI

Author: kddnewton

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "bundler"
+    directory: "/"
+    schedule:
+      interval: "daily"

.github/workflows/main.yml

@@ -0,0 +1,34 @@
+name: Main
+on:
+- push
+- pull_request_target
+jobs:
+  ci:
+    name: CI
+    runs-on: ubuntu-latest
+    env:
+      CI: true
+    steps:
+    - uses: actions/checkout@master
+    - uses: ruby/setup-ruby@v1
+      with:
+        bundler-cache: true
+        ruby-version: '3.1'
+    - name: Test
+      run: |
+        bundle exec rake test
+        bundle exec rake stree:check
+  automerge:
+    name: AutoMerge
+    needs: ci
+    runs-on: ubuntu-latest
+    if: github.event_name == 'pull_request_target' && github.actor == 'dependabot[bot]'
+    steps:
+    - uses: actions/github-script@v3
+      with:
+        script: |
+          github.pulls.merge({
+            owner: context.payload.repository.owner.login,
+            repo: context.payload.repository.name,
+            pull_number: context.payload.pull_request.number
+          })