Add pixi support (document how to install pixi in the shell)

[certik]

Either add some magic commands, or at least document how to install pixi from the shell on all platforms.

Then document how to use pixi to install vim and other essential programs that are not part of the shell itself.

[certik]

One one machine I installed pixi using rust by cargo b -r. Then I copied pixi.exe to ~/bin, and added ~/bin to $PATH.

I think we can simply append such paths in .shellrc (#95), just like on Linux, no need to modify global PATH.

[certik]

Ideally, you type pixi, which shell would recognize and ask to download it. It's just one binary, so it will download it and put it somewhere (to be figured out), so that it just works.

Once pixi is installed (single binary), one can use pixi to install everything else (eventually including things like conda, rust etc.).

Does it make sense to install pixi into ~/.pixi/bin, since this directory has to be in your $PATH anyway? See #69 (comment) where it would work.