Support WORKSPACE and MODULE.bazel builtins

[withered-magic]

The builtin.pb from the default ApiExporter is missing quite a few builtin definitions. This PR mostly solves the issue of missing types/docs in language rules, but doesn't solve the missing WORKSPACE and MODULE.bazel builtins. Do some investigation to see if the ApiExporter can be modified to also export these.

Unfortunately, the current schema doesn't support WORKSPACE or MODULE.bazel ApiContexts, so for now we can probably just expose these builtins in the BAZEL context.