[canvaskit] Break up helper.js into smaller files

This will hopefully make some things easier to find/understand.
I have a few more ideas for breaking up other parts.

Bug: skia:11203
Change-Id: Ia54c13fd6e3c897e04a737b258f6e77c50a1aed3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/356839
Reviewed-by: Nathaniel Nifong <nifong@google.com>
5 files changed