Generate gems RBIs for vendored gems

[Morriar]

Some projects use vendored gems, for example under the bundler/ directory.

Such gems don't appear in the Gemfile and thus Tapioca doesn't generate any RBI for them.

Since we generally exclude the vendored directory from type-checking, we're getting missing constants and methods.

We should find a way to make Tapioca generate RBI files for vendored gems.