This adds the `nixvimExtend` attribute to the generated standalone
derivation, this attribute takes a module as an argument and returns a
new standalone derivation with the initial module & the extended module
merged together.
This requires pulling quite a lot of code from nixpkgs in order to
make the `either` type work correctly, the effects can be seen for
example in the documentation of `ollama.promps` and `ollama.actions`
