While the `all-formats.nix` module allows to import all formats at once, it lacks an interface to customize formats or add formats.
This is fixed by adding the top-level option `formatConfigs` (attrsOf deferredModule).
All format modules created under `config.formatConfigs` are mapped so their outputs are available under `config.formats` which has also been moved to the top-level (previously `config.system.formats`).
Done:
- add option `formatConfigs`
- move option `system.formats` -> `formats`
- add test for customizing a format
