Make some attributes more typesafe

[mattesmohr]

This pull request tries to make some attributes more typesafe.

Vector attributes

// Absolute length
Rectangle {}.position(x: 50, y: 50)
Circle {}.center(x: 10, y: 10)
Rectangle {}.radius(x: 10, y: 10)
Vector {}.viewBox(x: 0, y: 0, width: 400, height: 200)

// Relative length
Circle {}.center(UnitPoint(x: 10, y: 10, format: .relative))

The challenge here is that these attributes accept absolute lengths such as 50 or 50.3 (for precision), as well as percentage values 50%. I couldn't come up with a better way to support both.

Also for future consideration, mixing those types (for whatever reason someone wants to do this) is not possible yet.

Circle {}.center(x: 10, y: 10.0) // This won't work

Sourceset attribute

Image()
   .sourceSet(SourceCandidate("...png", width: 1024), SourceCandidate("...png", width: 1680))

Image()
   .sourceSet(SourceCandidate("...png", density: .ultra))

Sizes attribute

Image()
   .sizes(SizeCandidate("15vw", conditions: .orientation(.landscape), SizeCandidate("100vw"))

Link()
   .sizes("16x16", "32x32")

Media attribute

Link()
   .media(MediaQuery(.screen, features: .orientation(.landscape)), MediaQuery(.print, features: .resolution("300dpi"))

Accept attribute

Input()
   .type(.file)
   .accept(.ogg, .mpeg)