v1.2.0: Adding the “fill_mode” property.

- Adding a `fill_mode` property in the `SvgSetup` that controls how the rooms are layout in the defined cavas.
- Adding the command line option `—fill-mode` to make the property accessible from the command line.
- Refactoring: Move code from the layout class into individual dimension classes like `Size` to make the code simpler and readable.
- Docs: Adding the documentation and examples for the new property.