More Xamarin to MAUI helpers

[davidortinau]

Can we get a lightbulb to change:

<OnPlatform x:Key="LittleSize" x:TypeArguments="x:Double" iOS="9" Android="9" />

with error:

Severity    Code    Description Project File    Line    Suppression State
Error   XLS0413 The property 'iOS' was not found in type 'OnPlatform`1'.    XamarinTV   C:\Users\daortin\source\repos\app-xamarintv\XamarinTV\Styles\DefaultTheme.xaml  106 

to:

<OnPlatform x:Key="LittleSize" x:TypeArguments="x:Double">
    <On Platform="iOS" Value="9"/>
    <On Platform="Android" Value="9"/>
</OnPlatform>

Can we fix these to update to WinUI?

{OnPlatform Default=18, UWP=8}

Severity    Code    Description Project File    Line    Suppression State
Warning XLS1111 'UWP' is obsolete: 'Use WinUI instead.'.    XamarinTV   C:\Users\daortin\source\repos\app-xamarintv\XamarinTV\Styles\DefaultTheme.xaml  74  

[mgoertz-msft]

Adding @etvorun for updating OnPlatform UWP => WinUI. Should just be a matter of adding a new mapping.