Add support for using VSCode and Metals for Java/Scala

[stuhood]

Now that Pants supports BSP for Java/Scala, adding support for Metals will involve implementing the remaining methods that Metals uses to index and run tests. See https://scalameta.org/metals/docs/integrations/new-build-tool.

The BSP protocol implementation in Pants has:

• a generic core
• a set of JVM assistance rules
• per-language rules for java and scala

[stuhood]

Metals uses DAP for test running, so #16216 will be an important component of this issue.