feat(alejandra): add settings option for indentation config

[anders130]

Adds settings options for the alejandra formatter, enabling configuration via a generated alejandra.toml.

Closes #426

Changes

• programs/alejandra.nix: add programs.alejandra.settings.indentation option (TwoSpaces | FourSpaces | Tabs, default null)

Usage

programs.alejandra = {
  enable = true;
  settings.indentation = "FourSpaces";
};