Split pixi.d.ts out of phaser.d.ts

`pixi.d.ts` has been added and is no longer a part of `phaser.d.ts`.  The latter now expects `pixi.d.ts` to be a sibling and uses a reference link.  Just keep the 2 files together, and reference only `phaser.d.ts` in your project as usual. The webgl.d.ts file is here but not linked to anything by default.

Not sure if a readme belongs here either. Open to advice.