Allows setting home.shellAbbrs that either get set as Fish shell
abbreviations or converted to shell aliases depending on if Fish is
active.
Abbrs that depend on Fish-specific features get automatically filtered
out.
Switch to nixfmt as when I was adding treesitter injected lang hints,
Alejandra was not formatting those comments in a way I liked. Turns out
I like the nixfmt style in general better.
