Parse NSFont.TextStyle correctly

[vancura]

As we have styles for headers and labels, it would be good to associate them with NSFont.TextStyle properties and not override font sizes and weights.

Related:

• https://github.com/netonjm/FigmaSharp/blob/f9a1ff71605485199250f71bd48b78dbb24909b1/FigmaSharp.Controls/FigmaSharp.Controls.Cocoa/Services/ControlTypeService.cs#L43
• https://github.com/netonjm/FigmaSharp/blob/b5ff5b28240e4413820f9b9dd5eba5c58ce2cd6a/FigmaSharp.Controls/FigmaSharp.Controls.Cocoa/Converters/Controls/LabelConverter.cs#L84
• https://github.com/netonjm/FigmaSharp/blob/b5ff5b28240e4413820f9b9dd5eba5c58ce2cd6a/FigmaSharp.Controls/FigmaSharp.Controls.Cocoa/Helpers/ViewHelper.cs#L42-L116
• https://developer.apple.com/documentation/appkit/nsfont/textstyle#

[hbons]

Since this is a (much welcomed) Big Sur feature, we should add a check here so that we can also keep supporting Catalina. In any case we need to wait for the Xamarin.Mac bindings to add this, so it may be a good idea to this the "old" way for now.