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

### 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.