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

flake/dev/tests.nix

@@ -0,0 +1,14 @@
+{
+  self,
+  helpers,
+  ...
+}:
+{
+  perSystem =
+    { pkgs, ... }:
+    {
+      checks = pkgs.callPackages ../../tests {
+        inherit helpers self;
+      };
+    };
+}