feat(experimental): recognise rocks-dev packages as dependencies

[mrcjkb]

Makes uses of the new experimental API for tricking luarocks into recognising packages managed by external modules as dependencies.

See:

• https://github.com/nvim-neorocks/rocks.nvim/issues/365

Requires the following experimental feature to be enabled in rocks.nvim:

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

Depends on: https://github.com/nvim-neorocks/rocks.nvim/pull/492