fix(wasm): Expose FontFamilyHelper through Uno.UI.Runtime.WebAssembly

Author: jeromelaban

doc/articles/features/custom-fonts.md

@@ -155,9 +155,11 @@ public static void main(string[] orgs)
     // You can add more than one, but preload too many fonts could hurt user experience.
     // IMPORTANT: The string parameter should be exactly the same string (including casing)
     //            used as FontFamily in the application.
-    Windows.UI.Xaml.Media.FontFamilyHelper.PreloadAsync("ms-appx:///Assets/Fonts/yourfont01.ttf#ApplicationFont01");
-    Windows.UI.Xaml.Media.FontFamilyHelper.PreloadAsync("https://fonts.cdnfonts.com/s/71084/antikythera.woff#Antikythera");
-    Windows.UI.Xaml.Media.FontFamilyHelper.PreloadAsync("Roboto");
+    Uno.UI.Xaml.Media.FontFamilyHelper.PreloadAsync("ms-appx:///Assets/Fonts/yourfont01.ttf#ApplicationFont01");
+    Uno.UI.Xaml.Media.FontFamilyHelper.PreloadAsync("https://fonts.cdnfonts.com/s/71084/antikythera.woff#Antikythera");
+
+    // Preloads a font which has been specified as a CSS font, either with a data uri or a remote resource.
+    Uno.UI.Xaml.Media.FontFamilyHelper.PreloadAsync("Roboto");
 
     Windows.UI.Xaml.Application.Start(_ => _app = new App());
 ```

src/Uno.UI.Runtime.WebAssembly/Xaml/Media/FontFamilyHelper.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Threading.Tasks;
+using Windows.UI.Xaml.Media;
+
+namespace Uno.UI.Xaml.Media;
+
+/// <summary>
+/// WebAssembly specific <see cref="FontFamily"/> helper
+/// </summary>
+public partial class FontFamilyHelper
+{
+	/// <summary>
+	/// Pre-loads a font to minimize loading time and prevent potential text re-layouts.
+	/// </summary>
+	/// <returns>True is the font loaded successfuly, otherwise false.</returns>
+	public static Task<bool> PreloadAsync(FontFamily family)
+		=> FontFamily.PreloadAsync(family);
+
+	/// <summary>
+	/// Pre-loads a font to minimize loading time and prevent potential text re-layouts.
+	/// </summary>
+	/// <returns>True is the font loaded successfuly, otherwise false.</returns>
+	public static Task<bool> PreloadAsync(string familyName)
+		=> PreloadAsync(new FontFamily(familyName));
+}