Default colors should respect theme

[Cannedfood]

Maui's native components automatically use a dark background and white text color by defaut if nothing is set.
I think this control should behave the same.

This currently leads to black-on-black text in dark mode.

As a workaround I currently use AppThemeBinding to set the text color:

<idk:MarkdownView
    ...
    TextColor="{AppThemeBinding Light=Black, Dark=White}"
/>