starship: add option to use community presets

[kpbaks]

Description

Presets are community-submitted configurations for starship. See: https://starship.rs/presets/#presets.

This PR adds the option programs.starship.preset that can be set to the name of a preset. If programs.starship.preset != null it will have priority over programs.starship.settings.

The preset is fetched from starships github repo by using pkgs.fetchFromGithub. I do not think this implementation detail is ideal, as the version fetched from the repo can be out of sync with programs.starship.package. This can lead to a bad user experience where a user has seen a preset online and wants to use it, but cannot because the revision is out of date. I am not experienced enough with nix and nixpkgs to know of an alternative. You can get the TOML of the preset with starship preset <name> where <name> is one of the items in starship preset --list.

Checklist

• Change is backwards compatible.

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