Implement JDT.LS extension for Kotlin files

[fwcd]

Addresses #4

The JDT language server can be extended to support custom project importers. Since Kotlin symbols cannot currently be referenced inside Java source files, we could provide such an extension in a subproject inside this repo with an importer that locates and registers Kotlin source files in the JDT's project model.

Implementation notes:

• A list of JDT.LS extension points can be found here
• An example JDT.LS extension can be found here
  • Note how the VSCode extension contains a custom contribution point for javaExtensions
• The JDT.LS importer should have a higher precedence that the default ones (see Best way to prefer GRADLE_{HOME,DIST_URI} over gradlew eclipse-jdtls/eclipse.jdt.ls#480 (comment))
• The importer could use the same class path/source path resolvers as the language server, possibly by sharing their implementations through the shared subproject
• See also: Support Kotlin gradle files? eclipse-jdtls/eclipse.jdt.ls#449, kotlin imports into Java file are unresolved redhat-developer/vscode-java#531

[jonathanmarvens]

@fwcd Is this currently being implemented? The VS Code Java/Kotlin interoperability story is pretty frustrating at the moment, so I appreciate your efforts on this!

[guidovalli]

news about this?

[oscarsan]

Any news about this? :-), about to change from intelliJ to visual studio, this is the only thing that is stopping me.