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.

build.gradle

@@ -30,6 +30,7 @@ allprojects {
         nanojsonVersion = "1d9e1aea9049fc9f85e68b43ba39fe7be1c1f751"
         spotbugsVersion = "4.6.0"
         junitVersion = "5.8.2"
+        checkstyleVersion = "9.3" // do not use latest version (10.0) as it requires compile SDK 11
     }
 }