build(nix): add flake.nix for cross-compilation from darwin and linux

I was getting really tired of having to switch between display inputs to
different platform-specific machines to be able to make and test changes
on komorebi for Windows and komorebi for Mac.

With this commit, the `flake.nix` provides a Nix devShell and crane
build for users to make and validate changes with `cargo check`, `cargo
clippy` and `cargo build` with the Windows MSVC toolchain on Linux and
macOS.

justfile

@@ -96,6 +96,14 @@ schemapub:
     npx wrangler pages deploy --project-name komorebi .\komorebi-schema
     npx wrangler pages deploy --project-name komorebi-bar .\bar-schema
 
+depcheck:
+    cargo outdated --depth 2
+    cargo udeps --quiet
+
+deps:
+    cargo update
+    just depgen
+
 depgen:
     cargo deny check
     cargo deny list --format json | jq 'del(.unlicensed)' > dependencies.json