Smaller color (4 bytes) to fit into 8 bytes for theming

[twop]

• introduced FabColor and converted all usecases
• FabColor Is going to live in Fabulous.XamarinForms and potentially copy pasted to MAUI when time comes, because it has a similar issue
• Made AppThemeValues require dark variant but in case dark and light are the same we only use SetValue, hence there is a new constraint 'T: equitable
• Added SmallScalar that is AppThemeValues<FabColor> that is encoded as two int32 fit into uint64 (4 bytes + 4 bytes)