wrappers/hm: add vimdiffAlias option

[GaetanLepage]

Implement the programs.neovim.vimdiffAlias option from HM in nixvim. Note: this only applies to the HM module.

Fixes #1351

[traxys]

This needs to be documented somewhere, because as is I don't think the option will appear in the documentation.

[traxys]

I though about this a bit more, I think we should add this feature in the same place as the viAlias/vimAlias and do the work for all the wrappers (nix-darwin, home-manager, nixos, standalone) or we should not do it at all

[MattSturgeon]

I think we should add this feature in the same place as the viAlias/vimAlias and do the work for all the wrappers (nix-darwin, home-manager, nixos, standalone)

Could we use pkgs.wrapProgram, pkgs.makeWrapper, or similar on platforms where we can't make shell aliases?

[traxys]

Yeah I think we would need to do this for the standalone wrapper. It may be the best to do it for everyone though (mabye even do this in nixpkgs proper? The viAlias & vimAlias is implemented there IIRC)

[MattSturgeon]

This needs to be documented somewhere, because as is I don't think the option will appear in the documentation.

This was resolved in #1355.

I think we should add this feature in the same place as the viAlias/vimAlias and do the work for all the wrappers (nix-darwin, home-manager, nixos, standalone)

Could we use pkgs.wrapProgram, pkgs.makeWrapper, or similar on platforms where we can't make shell aliases?

We already have defaultEditor exclusive to home-manager. Perhaps porting these could be handled in a separate PR?

I don't think not having this everywhere should necessarily be a blocker.

[MattSturgeon]

maybe even do this in nixpkgs proper? The viAlias & vimAlias is implemented there IIRC

That's correct. I'm happy for this to be merged as a stop-gap, but I agree long-term it'd be better to have this everywhere or even in nixpkgs' nvim wrapper.

IMO a lot of our wrapper stuff could be unified, so maybe there's some future (unrelated) refactoring to be done there...