[canvaskit] Add demo on how to decode images in web worker

Change-Id: I3c9107799c5df2c39a8ea8ddf106978786de79f7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/317389
Reviewed-by: Yegor Jbanov <yjbanov@google.com>
Reviewed-by: Nathaniel Nifong <nifong@google.com>
3 files changed