IDE support for JVM languages

[stuhood]

Although IDE support is useful for all of our supported languages, it's particularly necessary for Java (partially because it has historically been verbose, but also because IDEs for JVM languages are incredibly powerful).

Integrating with one or both of Jetbrains IDEA and VSCode will be important. It's possible that an LSP/BSP server implementation would work with both, although experts should be consulted on the degree to which targetting BSP would be limiting.

[stuhood]

The first priority we've heard from users is for IntelliJ, although support BSP should mean that both are supported. Whichever BSP integration is more debuggable is likely to be easiest to iterate with though.

[stuhood]

Preliminary support for using BSP to load Scala/Java projects is available in the latest 2.12.x releases! Further polish to that support is tracked in the M4 and M5 milestones of https://github.com/pantsbuild/pants/projects/22?fullscreen=true