Gazelle Language Extension

rules_jsonnet targets aren't too complicated and can be done by hand, but it would be nice to have gazelle do most of the heavy lifting, right?

We should create a rules_jsonnet language extension for gazelle, which would automatically create BUILD definitions for predictable things (like a jsonnet_library target if there is a file.libsonnet or a jsonnet_to_json if there is a file.jsonnet). I know imports are a bit more work, but we can always try to solve that later?

https://github.com/bazelbuild/bazel-gazelle/blob/master/extend.md#supported-languages

Start with Gazelle's extend.md section on supported languages, linked in the issue, and compare how language extensions generate BUILD definitions. Define the initial scope around predictable jsonnet_library and jsonnet_to_json targets for .libsonnet and .jsonnet files; imports are explicitly deferred. Done means the extension can generate those definitions through Gazelle.

Written by the indexing model from the issue text.