UI measurement tracking issue

[ickshonpe]

bevy_ui's measurement API is extremely neglected. It's poorly documented, hard to use, and there are persistant bugs that have stuck around for years.

Bugs

• The max_width/height constraints don't compose correctly with measure funcs.
  max_width/height constraints don't compose correctly with measure funcs #15543
• Node incorrectly sized with image child under specific conditions
  Node incorrectly sized with image child under specific conditions #17792
• Taffy crashes when UI hierarchy is modified in specific way
  Taffy crashes when UI hierarchy is modified in specific way 2 #20642
• Warn on mixing UI content types
  Emit warning when mixing node types on an entity #17466

Enhancements

• The Measure trait can't be implemented without importing taffy
  The Measure trait can't be implemented without importing taffy #21423
• Example demonstrating how to use the Measure trait.
  Custom measure func example #9687
• Measure funcs should be directly injected into UiSurface
  Inject measure funcs directly into UiSurface #17779