Allow the Decompiler (Fernflower) to also work on .jar files

[dasTanmoy0096]

According to FernFlower's documentation at https://github.com/JetBrains/intellij-community/tree/master/plugins/java-decompiler/engine, seems like it does support decompiling .jar files yet the Language Server doesn't seem to be able to process those files. I know I can just simply extract and decompile those files individually, but a jar viewer/decompiler would also be nice.

[rgrunber]

I think a similar thing was asked in #3160 . What client are you using with JDT-LS ? I think it's just a matter of moving some code into JDT-LS so we can properly handle jar files.

[dasTanmoy0096]

Currently using VSCode with JDT-LS, and it does mention in that issue that the Project Manager can look inside jars. But I believe it's only for dependency jars though? Not sure if it can decompile project target jars.

[rgrunber]

Yup, It only seems to do it for dependencies and the JRE. We'd have to figure out if we can represent jar files that aren't dependencies in the project's Java model. I don't think we'd want to add it to the classpath, but if we could just pass the URI and get some kind of Java model element, it should be possible. I'll mark this as a duplicate in favour of #3160 since once that's addressed, we could do this.

Also, we do support using Fernflower for arbitrary .class files. See redhat-developer/vscode-java#2985 (comment) . The issue with that approach is whether creating a custom editor page for a Jar file is really the right approach.