Add checkstyle to extractor gradle project

With respect to NewPipe's checkstyle.xml, checkstyle is disabled for javadoc comments. There is no need for strict rules over comments here in the extractor, as sometimes javadocs are just needed to clarify a small thing and having empty/meaningless @param or @throws is useless.

extractor/build.gradle

@@ -1,9 +1,25 @@
+plugins {
+    id 'checkstyle'
+}
+
 test {
-//  Pass on downloader type to tests for different CI jobs. See DownloaderFactory.java and ci.yml
+    // Pass on downloader type to tests for different CI jobs. See DownloaderFactory.java and ci.yml
     if (System.properties.containsKey('downloader')) {
         systemProperty('downloader', System.getProperty('downloader'))
     }
     useJUnitPlatform()
+    dependsOn checkstyleMain // run checkstyle when testing
+}
+
+checkstyle {
+    getConfigDirectory().set(rootProject.file("checkstyle"))
+    ignoreFailures false
+    showViolations true
+    toolVersion checkstyleVersion
+}
+
+checkstyleTest {
+    enabled false // do not checkstyle test files
 }
 
 dependencies {
@@ -15,6 +31,8 @@ dependencies {
     implementation "com.github.spotbugs:spotbugs-annotations:$spotbugsVersion"
     implementation 'org.nibor.autolink:autolink:0.10.0'
 
+    checkstyle "com.puppycrawl.tools:checkstyle:$checkstyleVersion"
+
     testImplementation platform("org.junit:junit-bom:$junitVersion")
     testImplementation 'org.junit.jupiter:junit-jupiter-api'
     testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine'