[MacosTextField] Introduce a `MacosTextFieldTheme` and `MacosTextFieldThemeData`

[GroovinChip]

[GroovinChip]

I've contemplated this further, and it occurs to me that the way the material library handles customization of TextField style is via InputDecoration. Perhaps instead of having a MacosTextFieldThemeData we should just do something similar? We'd have to do some refactoring, since we handle styling via various private variables.

What do you think @whiplashoo?

[whiplashoo]

You are probably referring to this implementation style?. As I understand, they merge an InputDecorationTheme and the constructor-provided decoration property, to get the final input decoration.

[GroovinChip]

Something like that, yeah.