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.

komorebic/Cargo.toml

@@ -41,4 +41,4 @@ default = ["schemars"]
 schemars = ["dep:schemars", "komorebi-client/default"]
 
 [lints.rust]
-unexpected_cfgs = { level = "warn", check-cfg = ['cfg(FALSE)'] }
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(FALSE)'] }