Types for registerWidget don't match the implementation

**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

Start at the registerWidget type declaration and implementation, then inspect the existing usages throughout the codebase. Done means the types represent all three registration forms supported by the implementation and those current usages type-check successfully.

Written by the indexing model from the issue text.