Update Monaco Uri class to use System.Uri or Windows.Foundation.Uri

[hawkerm]

Not sure which of the two .NET Uri classes makes more sense to use, but I think they both support the pieces needed to do a nice JsonConverter to make it easier to use the API in .NET.

[hawkerm]

We did use System.Uri already for the CssGlyphStyle here:

monaco-editor-uwp/MonacoEditorComponent/Monaco/Helpers/CssGlyphStyle.cs

Line 5 in b390c53

public System.Uri GlyphImage { get; set; }

[hawkerm]

Not sure if there's other places, but for the Monaco Model Uri, there's a different problem, filed here #59.

[hawkerm]

Yeah, the Monaco Uri is special for a couple of things, so not sure if this makes sense in some spots. Need to think about this more.