feat: add fish support to shell installers

[mistydemeo]

This adds support for fish as a shell. Like with sh, I took inspiration from rustup's implementation. This broadly leans on the existing shell support, but has a few distinct features.

• Instead of writing to an existing configuration file like ~/.bashrc, we write to a file in fish's conf.d path. This is a directory which is autoloaded on shell boot, which means we can create an app-specific path and can avoid having to edit the user's own configuration file.
• Unique to fish, since conf.d may not exist at the time we want to interact with it, I've added an mkdir -p call.
• I've used fish's fish_add_path helper, which encapsulates the logic of "only prepend to the path if it's not already on the PATH" without us having to write it ourselves. (Note that fish_add_path was introduced in fish 3.2.0, which was released in 2020. Since that version is available in the current LTS release of Ubuntu and Debian stable, I believe it's old enough to be safe for us to use it.)
• I updated our existing shell writing code to make it aware of what shell we're writing so I could branch which form of envfile to write, and whether to output source or . statements. (fish strongly prefers source and has stated . may be removed in the future.)

I gave this a test locally; it appears to be working fine, and, like with the sh side, doesn't overwrite cargo's configuration for CargoHome installs.

[Gankra]

Seems good, except for one weird thing.