Support for expressions

[xvrh]

Expressions for AfterEffects/Lottie are written in Javascript.
It would be very hard to support Javascript expression directly in this library. It requires to embedded a Javascript
engine for platforms that can't run javascript directly (android/iOS...) and we will need a way to interact synchronously with the engine.

This is a very difficult path that would have big impacts on the complexity, size and portability of this library.
Thus this library doesn't provide direct support for Lottie Expression.

It is possible to provide an alternative: we could support "Dart expression" and let the developer provides the conversion of the javascript function in Dart.

Example:
pub run lottie generate-expressions -o lib/lottie_expressions.dart path/to/lottie.json

// GENERATED FILE to modify

final expressions = {
  "layers['Name of the layer'].x": _layersNameOfTheLayerX,
};

/*
  HASH: abc1321321
  Original code:
  var $bm_rt;
  var temp;
  temp = div(content('Rectangle 1').content('Rectangle Path 1').size[0], 2);
  $bm_rt = [
      temp,
      0
  ];
*/
Offset _layersNameOfTheLayerX(Context context) {
  var content = context.content('Rectangle 1').content('Rectangle Path 1');
  return Offset(content.size.x, 0);
}

return Lottie.asset('my_animation.json', expressions: expressions);