feat: support for Fish on Windows

[fstanis]

This PR fixes #4648.

Currently, on Windows (via #3981) we launch bash by creating a new bash process with --init-file which contains all the environment variables, completions and custom prompt.

This doesn't work for other shells unfortunately since --init-file is bash-specific. This PR splits the init file into two parts: one for the environment variables (called init_file in code), which uses bash syntax, and the other one for completions and custom prompt, which is specific to the shell that's starting (called env_file in code).

pixi shell therefore first starts bash in a new process with the init_file, then one of two things happens:

• If we're using fish, we use exec for fish to "take over" the bash process, sourcing env_file in the process.
• If we're using bash, we simply source the file with env_file.

This logic should be possible to reuse for xonsh and zsh as well, although someone more knowledgeable would need to clarify how to best perform the "take over" step for those shells.

[mwiebe]

I wonder if you could leave the bash --init-file logic the same as before, and switch this -i into -c fish -i when the shell is fish?

[fstanis]

I tried that initially and, unfortunately, it didn't work. While fish inherits the environment variables from the parent bash, it can't inherit the functions (i.e. prompt and completions) because those are shell-specific. So either that would start fish without the prompt or shell completion (if we always use those for bash regardless of shell) or would fail completely (because fish function syntax is incompatible with bash).

[fstanis]

Thinking about it, we could probably just skip completions and prompt for zsh for now (or for fish as well, if we want to make this code simpler) and possibly tell the user how to add these manually by editing their .zshrc / config.fish.