Add shell command

[ruben-arts]

Adding the ability to run pixi shell next to pixi run x so people can activate the environment and run there commands in there.

[wolfv]

A good way to support more shells properly is to first run bash and then the target shell. That way all the activation scripts usually work.

E.g. bash -c "./activation.sh && exec $TARGET_SHELL where $TARGET_SHELL could be fish or xonsh...

[baszalmstra]

I think when using powershell this is very important! Not all packages support powershell.

[baszalmstra]

I get this error:

E:\Developer\prefix\pixi\myproject>$SHELL
'$SHELL' is not recognized as an internal or external command,
operable program or batch file.

[baszalmstra]

@ruben-arts and I just tested this one windows. It selected the proper shell both in powershell.exe, pwsh.exe and cmd.exe.