Add File.java to forbidden APIs

This commit cuts over all of core (not quite all tests) to java.nio.Path
It also adds the file class to the core forbidden APIs to prevent its usage
I dislike this idiom here, because if e.g. encoding is wrong it will silently replace with U+FFFD. Is this done anywhere else in the codebase? If so, maybe useful to have a method, that would e.g. set onMalformedInput/onUnmappableCharacter and so on.