flake: initial flake-parts modules

[MattSturgeon]

• flake: add initial flake-parts module
• flake: add nixvimConfigurations flake-parts module
• flake: add nixvimModules flake-parts module

This adds flake-parts modules for declaring flake.nixvimModules and perSystem.nixvimConfigurations. A default module imports both of these.

This is currently undocumented. It should be possible to document these modules in a similar way to how we document platform-wrappers. We may also want to document our flake-parts modules on the flake.parts website?

@traxys also proposed having some auto-configuration, which I agree is a good idea. However I'd like to keep the scope of this PR small, so let's postpone that for now.

[MattSturgeon]

Re documentation:

I've sent hercules-ci/flake.parts-website#1288 upstream that should allow us to leverage the flake-parts-website tooling without having to also have extra stuff in our flake's packages outupt.

It would require adding flake-parts-website as a flake input though, or fetching it with a fetcher I guess.

The alternative is to use flake-parts-lib.evalFlakeModule manually, but scrolling through render-module.nix there's a few edge-cases & botstrapping we may need to handle too if we take that approach.

[GaetanLepage]

Looks fine to me overall.

[MattSturgeon]

@mergify queue

[mergify]

queue

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The pull request has been merged automatically at 8fb2fe2

[mergify]

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