Ensure svg model has no trimmer warnings

[OleRoss]

Hi, first of all, thank you for this library.

I was running an AOT publish with a reference to Svg.Controls.Skia.Avalonia 2.0.0.13 which referenced Svg.Model 5.1.1 and got the following trimmer warnings:

    /_/src/Svg.Model/Services/TransformsService.cs(346): Trim analysis warning IL2026: Svg.Model.Services.TransformsService.TryResolveTransformOriginComponent(String,Boolean,SvgElement,SKRect,Single&): Using member 'System.ComponentModel.TypeDescriptor.GetConverter(Type)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Generic TypeConverters may require the generic types to be annotated. For example, NullableConverter requires the underlying type to be DynamicallyAccessedMembers All.
    /_/src/Svg.Model/Services/SvgGeometryService.cs(197): Trim analysis warning IL2026: Svg.Model.Services.SvgGeometryService.GetComputedUnit(SvgElement,String,SvgUnit,Boolean,Boolean&,Boolean&,Boolean): Using member 'System.ComponentModel.TypeDescriptor.GetConverter(Type)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Generic TypeConverters may require the generic types to be annotated. For example, NullableConverter requires the underlying type to be DynamicallyAccessedMembers All.

To fix this, I noticed that other code paths use SvgUnitConverter.Parse and decided to use this as a trimmer-safe alternative.

Additionally, I added AOT/Trimmer build properties to the relevant projects to ensure these errors are caught during normal builds in the future.

[OleRoss]

Thank you for merging!