Introduce GitHub Actions based CI workflow

This PR introduces a CI workflow using GitHub Actions and removes the Travis CI configuration file.

Find new workflow runs for this PR here: https://github.com/sormuras/google-java-format/actions

#### TODO

- [x] Email [Notifications](https://docs.github.com/en/free-pro-team@latest/github/managing-subscriptions-and-notifications-on-github/configuring-notifications) -- register `[email protected]` as a watcher or let each interested use register themself?
- [ ] Test publish snapshot job on `google:master` -- after storing credential [secrets](https://github.com/google/google-java-format/settings/secrets/actions).
- [ ] Remove AppVeyor [integration](https://github.com/google/google-java-format/settings/installations) and delete `appveyor.yml` configuration file.

Closes #543

Fixes #544

COPYBARA_INTEGRATE_REVIEW=#544 from sormuras:github-action a689f62
PiperOrigin-RevId: 344280060

.github/workflows/ci.yml

@@ -0,0 +1,58 @@
+name: CI
+
+on: [ push, pull_request ]
+
+jobs:
+  test:
+    name: "JDK ${{ matrix.java }} on ${{ matrix.os }}"
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ ubuntu-latest, macos-latest, windows-latest ]
+        java: [ 15, 11 ]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - name: 'Check out repository'
+        uses: actions/checkout@v2
+      - name: 'Cache local Maven repository'
+        uses: actions/cache@v2
+        with:
+          path: ~/.m2/repository
+          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+          restore-keys: |
+            ${{ runner.os }}-maven-
+      - name: 'Set up JDK ${{ matrix.java }}'
+        uses: actions/setup-java@v1
+        with:
+          java-version: ${{ matrix.java }}
+      - name: 'Install'
+        shell: bash
+        run: mvn install -DskipTests=true -Dmaven.javadoc.skip=true -B -V
+      - name: 'Test'
+        shell: bash
+        run: mvn test -B
+
+  publish_snapshot:
+    name: 'Publish snapshot'
+    needs: test
+    if: github.event_name == 'push' && github.repository == 'google/google-java-format' && github.ref == 'refs/heads/master'
+    runs-on: ubuntu-latest
+    steps:
+      - name: 'Check out repository'
+        uses: actions/checkout@v2
+      - name: 'Cache local Maven repository'
+        uses: actions/cache@v2
+        with:
+          path: ~/.m2/repository
+          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+          restore-keys: |
+            ${{ runner.os }}-maven-
+      - name: 'Set up JDK 15'
+        uses: actions/setup-java@v1
+        with:
+          java-version: 15
+          server-id: sonatype-nexus-snapshots
+          server-username: CI_DEPLOY_USERNAME
+          server-password: CI_DEPLOY_PASSWORD
+      - name: 'Publish'
+        run: mvn source:jar deploy -B -DskipTests=true -Dinvoker.skip=true -Dmaven.javadoc.skip=true