Types for registerWidget don't match the implementation

[fgnass]

Describe the bug

The types for registerWidget only specify a single signature, but the implementation allows two additional ways of registering widgets, which are both actively used throughout the codebase.

Why this matters

Currently, decap-cms-app registers all widgets eagerly at once, which results in very large bundle sizes. I would like to provide an alternative, more modular approach, but this requires the types to match in the first place.

What to do

Stay tuned for a PR or possibly an update to #7402