Support ThemeExtension

[fischerscode]

Material supports ThemeExtensions. I think it would be nice if fluent_ui would provide something similar. The implementation seems to be not that difficult.

In particular, I'd like to use theme_tailor in my fluent_ui project.

Is something like this considered to be out of scope for fluent_ui or might a PR get accepted?

[bdlukaa]

On the latest versions, you can use FluentThemeData.extensions, which has a similar API to material's ThemeData.extension