flake: partition dev dependencies

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

tests/no-flake.nix

@@ -9,7 +9,7 @@ let
   nixvim' =
     (import (
       let
-        lock = builtins.fromJSON (builtins.readFile ../flake.lock);
+        lock = builtins.fromJSON (builtins.readFile ../flake/dev/flake.lock);
       in
       fetchTarball {
         url =