This removes the need for end-users to manually set
`nixvim.inputs.devshell.follows = ""` (etc)
We offload evaluation of some of our flake modules into a `dev`
partition submodule.
- When its not needed, this submodule is not evaluated.
- When it is needed, it fetches extra inputs from `flake/dev/flake.nix`
as part of evaluating the submodule.
See https://flake.parts/options/flake-parts-partitions.html
This template uses the lower-level `evalNixvim` function to work
directly with nixvim configurations, instead of the prior idiom of using
`makeNixvim` to produce nixvim packages.
Additionally, the template is build on flake.parts, and demonstrates
using our recently added flakeModules.