add GitHub Actions CI

Author: sullis

.github/workflows/cross-version.yml

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+name: Cross-Version
+
+on: [push, pull_request]
+
+jobs:
+
+  test_java:
+    name: Java ${{ matrix.java }}
+    strategy:
+      fail-fast: false
+      matrix:
+        java: [14, 15, 16-ea]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Setup Java
+        uses: actions/setup-java@v1
+        with:
+          java-version: ${{ matrix.java }}
+      - name: Cache Maven Repository
+        uses: actions/cache@v2
+        with:
+          path: ~/.m2
+          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+          restore-keys: ${{ runner.os }}-maven
+      - name: Test
+        run: ./mvnw -V --no-transfer-progress -e clean verify javadoc:javadoc

.github/workflows/main.yml

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+name: CI
+
+on: [push, pull_request]
+
+jobs:
+
+  test_os:
+    name: OS ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macOS-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v2
+      - name: Setup Java
+        uses: actions/setup-java@v1
+        with:
+          java-version: 11
+      - name: Cache Maven Repository
+        uses: actions/cache@v2
+        with:
+          path: ~/.m2
+          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+          restore-keys: ${{ runner.os }}-maven
+      - name: Test
+        run: ./mvnw -V --no-transfer-progress -e clean verify javadoc:javadoc
+
+  sonar:
+    name: Sonar code analysis
+    runs-on: ubuntu-latest
+    if: github.repository == 'assertj/assertj-core' && github.event_name == 'push'
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - name: Setup Java
+        uses: actions/setup-java@v1
+        with:
+          java-version: 11
+      - name: Cache Maven Repository
+        uses: actions/cache@v2
+        with:
+          path: ~/.m2
+          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+          restore-keys: ${{ runner.os }}-maven
+      - name: Test with Sonar
+        run: >
+          ./mvnw -V --no-transfer-progress -e clean verify javadoc:javadoc
+          org.sonarsource.scanner.maven:sonar-maven-plugin:3.7.0.1746:sonar
+          -Dsonar.host.url=https://sonarcloud.io
+          -Dsonar.organization=assertj
+          -Dsonar.projectKey=joel-costigliola_assertj-core
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          SONAR_TOKEN: ${{ secrets.SONAR_TOKEN }}