Added a new RichEditToolbar control that works in a comparable way to the InkToolbar for the InkCanvas control.

Here's an example usage for Windows.

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
        <RowDefinition Height="*" />
    </Grid.RowDefinitions>
    
    <made:RichEditToolbar x:Name="RichEditToolbar"  TargetRichEditBox="{x:Bind RichEditBox}">
        <made:RichEditToolbar.CustomOptions>
            <AppBarSeparator />
            <AppBarButton x:Name="SaveButton" Icon="Save" Label="Save" />
            <AppBarButton x:Name="UndoButton" Icon="Undo" Label="Undo" />
            <AppBarButton x:Name="RedoButton" Icon="Redo" Label="Redo" />
        </made:RichEditToolbar.CustomOptions>
    </made:RichEditToolbar>

    <win:RichEditBox x:Name="RichEditBox" Grid.Row="1" />
</Grid>

This results in the following UI.

Unfortunately, Uno Platform doesn't currently support the use of RichEditBox. The control has been custom built now to structure around the unsupported direct attachment to a RichEditBox exposing events that allow a developer to use the control in a different context in Uno Platform applications.

This PR also introduces a new LightenColorBrushConverter and DarkenColorBrushConverter for Uno Platform applications to take advantage of the color styling extensions available in the UI.Styling library.

PR checklist

• Samples have been added/updated (where applicable)
• Tests have been added/updated (where applicable) and pass [N/A]
• Documentation has been added/updated for changes
• Code styling has been met on new source file changes
• Contains NO breaking changes

Other information