implement java sourcecode or jvm-bytecode backend

bazel makes it pretty easy to use its java rules, so if we compile bosatsu to java code, it should be fairly easy to get a bazel rule.

The issue proposes a Bosatsu backend that emits Java source code or JVM bytecode, with a possible Bazel Java rule. First determine which target representation and integration approach are intended; done means Bosatsu programs compile successfully through that backend and can be built with the corresponding Bazel support.

Written by the indexing model from the issue text.