More specific overloads for OffscreenCanvas.getContext

[jtbandes]

I have a project which previously used @types/offscreencanvas for OffscreenCanvas support. That library defines separate overloads for the different getContext types:

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/a6b57ed5f511b6d1c842b6acb4961c966d4afb00/types/offscreencanvas/index.d.ts#L46-L58

When pulling in the new changes via microsoft/TypeScript#51300 in TS 4.9.4, it seems like there's only one definition of getContext, so my calls to e.g. getContext("2d")?.clearRect(...) now fail to compile:

Property 'clearRect' does not exist on type 'CanvasRenderingContext2D | OffscreenRenderingContext'.
  Property 'clearRect' does not exist on type 'ImageBitmapRenderingContext'.ts(2339)

It would be nice to use the same approach of overloading based on the contextType parameter so that the proper types can be used without an as downcast.

[StenAL]

I added these overloads in #1474, try using @types/web >= 0.0.88 for your DOM lib