feat(experimental): API for installing rockspec dependencies

[mrcjkb]

A follow-up to #492.
Closes #388.

This will allow rocks-git.nvim and rocks-dev.nvim to pass in dependencies (e.g. from a -scm-1 or -dev-1 rockspec in the repo root) to be installed with the luarocks stub.

Requires the following experimental feature to be enabled:

vim.g.rocks_nvim = {
  -- ...
  experimental_features = {
    "ext_module_dependency_stubs",
  },
}

and up-to-date rocks.nvim extensions.

[mrcjkb]

Self-merging, as this is trivial and disabled by default.