measure_size

• Fix onChange callback to emit non-nullable value

• Update flutter_lints to version 6.0.0
• Update demo.gif

• Added MeasureSizeBuilder using prototype elements to measure the sizes of multiple widgets with custom constraints.
• MeasureSize now uses a SingleChildRenderObjectWidget to measure size instead of the build method of a StatefulWidget.

• Added MeasureSizeRenderObjectWidget as alternative way to detect size changes outside of build calls.

• Fix Flutter SDK constraints
• Fix package description

• Updated for Flutter 3
  • New lints
  • Fixed AndroidManifest.xml in example
  • Remove unnecessary null-check

Changed

• Migrated to null-safety

• Initial release