[canvaskit] Refactor Canvas2D JS into own files

Rather than one monolithic file, we now have one monolithic
file (canvascontext2d) and several smaller files (one per class,
and some helpers).

This should make the code navigation a little easier.

Bug: skia:
Change-Id: Ia191c2db778591af21d2a6126f053c17c4f677f1
Reviewed-on: https://skia-review.googlesource.com/c/175996
Reviewed-by: Florin Malita <fmalita@chromium.org>
14 files changed