feat: add .mvn/extensions.xml to Maven cache key pattern

Closes #990

Maven build extensions declared in `.mvn/extensions.xml` can introduce
additional plugin dependencies (e.g. lifecycle participants, custom
packaging types). Including this file in the cache key hash ensures that
changes to extensions — which affect what plugin JARs Maven downloads —
properly invalidate the cache, preventing stale caches from missing
newly-required plugin dependencies.

Changes:
- src/cache.ts: add `**/.mvn/extensions.xml` to Maven pattern array
- __tests__/cache.test.ts: update pattern expectations; add new test
- README.md: document the new file in the Maven cache key hash list

src/cache.ts

@@ -28,7 +28,11 @@ const supportedPackageManager: PackageManager[] = [
       join(os.homedir(), '.m2', 'wrapper', 'dists')
     ],
     // https://github.com/actions/cache/blob/0638051e9af2c23d10bb70fa9beffcad6cff9ce3/examples.md#java---maven
-    pattern: ['**/pom.xml', '**/.mvn/wrapper/maven-wrapper.properties']
+    pattern: [
+      '**/pom.xml',
+      '**/.mvn/wrapper/maven-wrapper.properties',
+      '**/.mvn/extensions.xml'
+    ]
   },
   {
     id: 'gradle',