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.

komorebi-gui/Cargo.toml

@@ -13,4 +13,4 @@ egui_extras = { workspace = true }
 random_word = { version = "0.5", features = ["en"] }
 serde_json = { workspace = true }
 windows-core = { workspace = true }
-windows = { workspace = true }
+windows = { workspace = true }