The way the `bat` module is currently written makes it essentially
impossible to use themes and syntaxes without IFD, since you must
provide the contents as string, instead of just giving a path to be
linked.
With this change, setting themes/syntaxes by-string will start issuing
warnings, and a new attribute model is added, lightly inspired by how
`programs.zsh.plugins` avoided this issue.
This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.