[canvaskit] Add Initial Typescript definitions/types.

Medium-term goal is to contribute to https://github.com/DefinitelyTyped/DefinitelyTyped

Bug: skia:10717
Change-Id: I5941e51cd384778240c5cffd0baad3a2bdb06f2a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/320257
Reviewed-by: Nathaniel Nifong <nifong@google.com>
9 files changed