Added Maven workflow to validate pull requests and commits (#4432)

* Create maven workflow

* Also run it when the workflow file was modified

* Added local Maven cache

* I know how to count :eyes:

.github/workflows/maven.yml

@@ -0,0 +1,51 @@
+# This workflow automatically tests new commits and pull requests as they come in.
+# Note that this does not upload any artifacts, you will need to compile mcMMO manually
+# if you wish to create the actual jar.
+name: Compile and test
+
+on:
+  # We run our tests whenever the pom or a source file was touched.
+  # There is no need to run Maven when only the changelog was touched.
+  # We may also want to re-run this workflow when the workflow file itself
+  # was updated too.
+  push:
+    paths:
+    - 'src/**'
+    - 'pom.xml'
+    - '.github/workflows/maven.yml'
+
+  # Whenever someone submits a new pull request which modified the pom or a source file,
+  # we want to ensure it compiles successfully and that all tests will pass.
+  pull_request:
+    paths:
+    - 'src/**'
+    - 'pom.xml'
+
+jobs:
+  compile:
+    name: Maven compiler
+    runs-on: ubuntu-latest
+    steps:
+
+    # 1. Check out the current working tree
+    - name: Checkout repository
+      uses: actions/checkout@v2
+
+    # 2. Setup Java 1.8 JDK
+    - name: Java 1.8 setup
+      uses: actions/setup-java@v1.4.3
+      with:
+        java-package: jdk
+        java-version: 1.8
+
+    # 3. Setup local Maven package cache to speed up building
+    - name: Cache Maven packages
+      uses: actions/cache@v2
+      with:
+        path: ~/.m2
+        key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
+        restore-keys: ${{ runner.os }}-m2
+
+    # 4. Build via Maven 
+    - name: Build via Maven
+      run: mvn verify -B --file pom.xml