Adding CI workflow for building and testing with Java 21 on Linux, Windows, and macOS (#69)

Author: Muta-Jonathan

.github/workflows/standalone-ci.yml

@@ -0,0 +1,63 @@
+name: Standalone Test CI
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build-linux:
+    name: on Linux (Java 21)
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          java-version: '21'
+          distribution: 'temurin'
+
+      - name: Build and Test with Maven
+        run: mvn clean install
+
+  build-windows:
+    name: on Windows (Java 21)
+    runs-on: windows-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          java-version: '21'
+          distribution: 'temurin'
+
+      - name: Build and Test with Maven
+        run: mvn clean install
+
+  build-macos:
+    name: on macOS (Java 21)
+    runs-on: macos-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          java-version: '21'
+          distribution: 'temurin'
+
+      - name: Build and Test with Maven
+        run: mvn clean install