Add support for text shadows. - #22449

See flutter-team-archive/engine#6385 for Engine-side changes.

Tests will not pass until the engine is landed and rolled.

This is still missing golden tests, but I wanted to do that in a separate patch to keep the merging of this one smooth.

Should this be called TextShadow instead of Shadow?

If you are referencing the class Shadow, the idea is that Shadow is a base class of all sorts of faked shadows, and is not particularly specific to text. Other classes can presumably make use of it if they wished.

If you mean the 'shadows' property of TextStyle, since the "list of shadows" concept can be applied on multiple widgets, in order to keep it consistent and not have a Shadows prefix to each instance of shadows, I have called it just "shadows". I think this more unified API is a bit more predictable to work with. The fact that it is a Text shadow is signified by it being a property of TextStyle.