Update fill type to accept CanvasGradient

[squoddam]

Currently fill property, defined in ShapeConfig, only allows for strings to be passed.
But despite that it is actually possible to pass CanvasGradient to the fill, and it renders correctly (codesandbox demo)

This PR adds CanvasGradient as one of possible values of fill, the same way it is done for stroke:

konva/src/Shape.ts

Line 60 in 68b4ea3

stroke?: string | CanvasGradient;

[lavrton]

Good. Can you also update the shape.fill() method type?

[lavrton]

Thanks. I will make a release a bit later.