Add type information to builtins

[cameron-martin]

#8 added autocomplete to builtins but typed everything as any. We can do better than this since there's a bunch of type information in the protobuf.

[cameron-martin]

It isn't that obvious to me how to construct non-primitive values of starlark::typing::Ty.

It looks like the only option is to use Ty::starlark_value, which would require generating instances of StarlarkValue from the protobuf.

[hauserx]

On related note, there is bazelbuild/bazel#21979 which aims to add types to the stardoc output protos for builtin functions.