Add a simple nix dev shell

[srid]

[Feel free to reject this PR if you do not wish to have Nix support upstream]

This PR adds a simple flake.nix providing a development shell for people that use the Nix package manager. It provides the benefit of having everything setup automatically for Dioxus development -- all I have to do is open VSCode in the dev shell.

The flake provides only dev shell, and not the full Nix packages, because that will require a Cargo.lock. I originally wanted to write this flake to create the dioxus-cli package, but figured the dev shell would be useful to others.

[srid]

Note: if you choose to merge this, I'm willing to be the maintainer of this flake, and can even add a CI action for it.

[jkelleyrtp]

Is it all possible to move the flake/lock into a "dot" directory to keep the main folder less cluttered?

We like to add support for various shells/systems but they do end up cluttering the top level so I'm trying to keep things somewhat tidy.

[srid]

It is technically possible -- and I can make that change if you absolutely want -- but this practice is almost never practiced in any projects that use Nix. Having a top-level flake.nix (which is equivalent to having a top-level Cargo.toml for Rust workspaces) also signals to potential contributors that this project is Nix-friendly.

[jkelleyrtp]

Thanks @srid ! Sorry this sat here for so long.