[ENHANCEMENT] [jvm] Add --java-lib-sources argument to support JavaDocs

I'm wondering if it would be possible to add the ability for VSCode to display JavaDocs for extern classes provided by `--java-lib-extern` in the current build context.

I believe this would have to involve the ability to include and reference a sources JAR. I don't know if providing a sources JAR could improve the generation of `--dump`s as well.

Currently, the only way to provide documentation for external classes is to manually create `extern` classes which `@:native` to the desired class, and then document those. This is a large amount of work for larger libraries and is mostly redundant when Haxe can generate externs automatically, and when JavaDocs already exist.