Improve public Parse/ToString APIs of our primitive types

[maxkatz6]

Is your feature request related to a problem? Please describe.

Primitive types:

• Color, HslColor, HsvColor
• CornerRadius, Thickness
• Point, Size
• Vector, Vector3D
• GridLength
• RelativeScalar, RelativePoint, RelativeRect
• PixelPoint, PixelSize, PixelRect
• Cue

Enum-like types that should at least have IParsable only:

• Easing
• Cursor
• TextTrimming
• TextDecorationCollection
• ThemeVariant
• WindowTransparencyLevel

Other:

• BoxShadow, BoxShadows, Effect
• Matrix
• KeySpline?

Not primitives, but can be revisited as well:

• StreamGeometry, Geometry,
• TransformOperations, Transform

Describe the solution you'd like

1. [Must] StringTokenizer needs to use ReadOnlySpan<char> and ideally be a "ref struct" (breaking change? for some reason this class is not internal).
2. [Must] All primitive types have to implement IParsable and IFormattable.
3. [Should] Primitive types should implement ISpanParsable and ISpanFormattable.
4. [Should] All primitive parsting/formatting should respect culture info and culture specific separators. WPF does this already.
5. [Should] Make devtools use new TryParse methods.
6. [Optional] Primitive types should have TypeConverter registered on them. WPF and MAUI do that, but we gave up on the half-way - primarily because we don't really use them, and instead parse compile-time everything, which is better by itself.

Describe alternatives you've considered

No response

Additional context

Related #16389