Adding type definitions for L.pixiOverlay

[FreeJ1nG]

Hi!

I had some need in my company to use this library to render a bunch of markers, it looks like this whole project was fully made with javascript, so I wrote some type definitions for the L.pixiOverlay extension

Here is the code

I'm fully open for feedbacks and wether it is a good idea to create a PR to provide type definitions :)

Cheers!