feat(experimental): install rockspec dependencies

[mrcjkb]

See also: https://github.com/nvim-neorocks/rocks.nvim/pull/527

Requires an up-to-date rocks.nvim with the following experimental feature enabled:

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