Add GH actions to build example mod

.github/workflows/build.yml

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+# Automatically build the project and run any configured tests for every push
+# and submitted pull request. This can help catch issues that only occur on
+# certain platforms or Java versions, and provides a first line of defence
+# against bad commits.
+
+name: build
+on: [pull_request, push]
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        # Use these Java versions
+        java: [
+          1.8,  # Minimum supported by Minecraft
+          11,   # Current Java LTS
+          15    # Latest version
+        ]
+        # and run on both Linux and Windows
+        os: [ubuntu-20.04, windows-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - name: checkout repository
+        uses: actions/checkout@v2
+      - name: setup jdk ${{ matrix.java }}
+        uses: actions/setup-java@v1
+        with:
+          java-version: ${{ matrix.java }}
+      - name: make gradle wrapper executable
+        if: ${{ runner.os != 'Windows' }}
+        run: chmod +x ./gradlew
+      - name: build
+        run: ./gradlew build

.github/workflows/validate-gradle-wrapper.yml

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+# Ensure the gradle wrapper in the repository has not been tampered with.
+# If this check fails, something is seriously wrong -- try creating a new
+# wrapper from a local gradle install.
+
+name: validate gradle wrapper
+
+on: [pull_request, push]
+
+jobs:
+  build:
+    runs-on: ubuntu-20.04
+    steps:
+      - name: checkout repository
+        uses: actions/checkout@v2
+      - name: validate gradle wrapper
+        uses: gradle/wrapper-validation-action@v1