Add SVGOverlay component

[fenech]

Mentioned in #624, this component exists in Leaflet but has no analogue in this library.

A lot of code is duplicated with MapLayer - the reason for this is to give the component access to the SVG element that it creates (this.container). An alternative could be to create the DOM element in the constructor and call super({ ...props, container }).

Added docs and example.

[PaulLeCam]

Thanks!
I think creating the container in the createLeafletElement() method is a good way.
Could you please address the issues raised in CI so this can be merged?

[fenech]

@PaulLeCam updated to fix flow errors and amended the commit. I typed the container as Element because it looks like flow doesn't have proper types for SVG elements.

[PaulLeCam]

Thanks!