Run GitHub Actions for pull requests

This does not archive any artifacts, simply shows whether the build passes or fails.

.github/workflows/pull_request.yml

@@ -0,0 +1,18 @@
+name: Build Purpur
+on: [ pull_request ]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up JDK
+        uses: actions/setup-java@v2
+        with:
+          distribution: 'adopt'
+          java-version: '15'
+      - name: Configure Git
+        run: git config --global user.email "no-reply@github.com" && git config --global user.name "Github Actions"
+      - name: Apply patches
+        run: ./gradlew applyPatches
+      - name: Build
+        run: ./gradlew build