Support using the Accent Color for `cursorColor`, `selectionBackground`, etc.

Can there be support for the Accent Color brushes, so if I were to set my cursor or selction to my Accent Color, as it changes, that change will be reflected in Windows Terminal?

So that might need to be it's own separate feature request. The cursor and the selection are drawn in DirectX down in TermControl, so they work differently than a lot of the things in this spec (which are all XAML elements).

For those, we'd need to accept something like "cursorColor": "accent". When we did that, CascadiaSettings would know that the string isn't a "#rrggbb" color, so it could look up the color when we're building the TerminalSettings to pass to the TermControl. However, after that point, if the user changes their accent color, that wouldn't be reflected dynamically in the Terminal, until the user manually refreshes their settings.

It's not a horrible idea, but probably something we should move to it's own thread for discussion, since I don't think it really fits in with a lot of the other things in this spec.

Originally posted by @zadjii-msft in https://github.com/microsoft/terminal/pull/5772#issuecomment-624908257

#6338 is related.