How can I help to get this merged?

Beautiful work!

Some comments I have:

1. annotationMode must be documented.
2. Having more unit tests would be great. I'll gladly help out.
3. This technically deprecates renderForms in favor of much more powerful annotationMode, so renderForms is to be marked as deprecated (this one can be on me)

About (3) deprecating renderForms: we actually need to keep both renderForms and annotationMode as they serve distinct purposes.

1. renderForms controls the HTML layer.
2. annotationMode determines how to draw form/annotation on the canvas layer.

Example: to show filled non-editable input

{
  renderForms: false,
  annotationMode: ENABLE_STORAGE,
}