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.
This commit applies 'cargo fix --edition' to safely migrate the project
to Edition 2021 of Rust.
A rustfmt.toml has also be added to enforce the flattening of use
statements when running 'cargo fmt'.
