feat(experimental): a way to recognise plugins managed by extensions as dependencies

[mrcjkb]

Closes #365

This provides a function that extensions can invoke to install or remove a luarocks stub, in order to trick luarocks into thinking a luarocks package is installed.

That way, plugins managed by extensions like rocks-git or rocks-dev can be recognised by luarocks as being installed, if other luarocks packages depends on them.

TODO:

• Use the API in rocks-git and test it manually.
• Use the API in rocks-dev and test it manually.

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]

Done and tested.

We could later add the ability to add dependencies to the stub template; see #388

[mrcjkb]

come to think of it, we should probably put this behind a config flag and make it an experimental feature, with an announcement calling for testers.

[mrcjkb]

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