Use language=bash when shebang line uses zsh

[throwaway-helix-zsh]

This PR makes the editor use language=bash when the shebang line uses zsh. We already use language=bash for zsh related files (~/.zshrc, ~/.zshenv etc.), so it's expected for the same to be true for shebang lines as well.

[throwaway-helix-zsh]

@the-mikedavis Thanks for triggering the pipelines. This is ready to merge :)

[the-mikedavis]

Thanks!