shellenv.sh / fish shell: Move Brew PATHs to front if they exist (add -m arg to fish_add_path)

[WinkelCode]

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This PR addresses an issue when a PATH variable with Homebrew paths is inherited, modified, and then brew shellenv is evaluated again. In this case, the expected behavior is that Homebrew directories should be placed at the front of the PATH. However, since -m (move component) isn’t specified, fish_add_path doesn’t modify the variable, this can, for example, cause binaries in /usr/bin (like XCode CLI tools' Python) to take precedence.

I encountered this in VSCode, where eval (/opt/homebrew/bin/brew shellenv fish) in my config.fish didn’t properly update the PATH, as VSCode's environment resolution already included the paths, just not at the top. This change ensures $HOMEBREW_PREFIX/bin and $HOMEBREW_PREFIX/sbin are always placed first in PATH.

Correction:

It's not actually VSCode's fault that the Brew PATH components aren't at the top, they are, but when fish thinks it's a login shell (which it does when run in VSCode's terminal), it will add some PATH components before the inherited ones, causing the issue. Nonetheless, based on the behavior for other shells, it seems like the behavior with -m specified, which is what this PR does, would be what is intended for brew shellenv.

[MikeMcQuaid]

This seems consistent with what we do for other shells 👍🏻

[carlocab]

Thanks!

[branchvincent]

👍 agree with this as well!