Add initial GitHub Actions workflow

Signed-off-by: Graham <gpe@openrs2.org>

.github/workflows/build.yaml

@@ -0,0 +1,15 @@
+---
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-java@v3
+        with:
+          distribution: temurin
+          java-version: 11
+      - uses: gradle/wrapper-validation-action@v1
+      - uses: gradle/gradle-build-action@v2
+        with:
+          arguments: build