Refactor @wordpress/element out of custom.d.ts

[kevinfodness]

Description

The custom.d.ts file uses @wordpress/element instead of react as a source and should be updated to use react directly.

Use Case

Since the WordPress project has moved away from using @wordpress/element, we should too.

Acceptance Criteria

• Update custom.d.ts in the root of the repo to use react instead of @wordpress/element and ensure type checks pass after this change has been made.