experimental configuration-based nixvim template

[MattSturgeon]

Adds an experimental flake template that configures Nixvim directly using evalNixvim rather than the current standalone functions.

Related issues:

• Re-imagined standalone mode (nixvimConfig) #2210
• flake: initial flake-parts modules #2858

Todo:

• Document new template/approach
• A flake-parts module for perSystem.nixvimConfigurations
• Document new flake-parts module
• Ensure all template's checks are inherited into the main flake's checks
• Make evalNixvim easier to access (top-level aliases)
• Include nixvim-print-init in build.package
• Include man-docs in build.package

Since this is experimental/draft, we don't need to tick off everything above before merging.

[GaetanLepage]

This looks very promising !

[traxys]

As we are defining a flake parts module could we make it such that it automatically adds a package.${system}.nvim entry set to config.nixvimConfigurations.default.config.build.package and a check.${system}.nvim set to config.nixvimConfigurations.default.config.build.test if they exist?

We could add flake module options to disable this behaviour as needed. It's maybe a bit too magic?

Alternatively we could add nvim-${name} entries for each configuration in config.nixvimConfigurations

[MattSturgeon]

I'm moving the flake-parts module stuff to a dedicated PR: #2858