* repo: add shell.nix and .envrc
Makes usage on NixOS easier, by not having to drop into a hurry of
`nix-shell -p lots of packages` every time someone wants to experiment
around with wgpu.
The .envrc causes shell.nix to be eval'd automatically run iff
1. direnv is installed
2. the user `direnv allow`ed the shell.nix
* repo(shell.nix): add explanations and remove cruft
