Fully support Flutter export

[guidezpl]

Is your feature request related to a problem? Please describe.
This issue tracks Flutter support for export. The export currently exports a light and dark color scheme.

Describe the solution you'd like

• Export of custom colors
• Support for color harmonization
• Generating something similar to https://github.com/material-foundation/material-dynamic-color-flutter/blob/main/example/lib/complete_example.dart#L11-L99

Additional context
Support for theme extensions was added in flutter/flutter#98033
The canonical example is in the dynamic_color package https://github.com/material-foundation/material-dynamic-color-flutter/blob/main/example/lib/complete_example.dart

Related to #34