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/package-tests.nix

@@ -0,0 +1,8 @@
+{
+  perSystem =
+    { config, ... }:
+    {
+      # Test that all packages build fine when running `nix flake check`.
+      checks = config.packages;
+    };
+}