Improve types for builtin editor widget IDs

[nib-bturner]

Summary

Currently its pretty easy to mistype or put the wrong widgetId when changing editor interfaces. This PR introduces function overloading to the index.d.ts file to restrict this from string to the widget IDs listed in your website:
https://www.contentful.com/developers/docs/extensibility/app-framework/editor-interfaces/.

Description

Motivation and Context

I'm sick of guessing the wrong widgetId and would prefer a nicer developer experience. Copying these changes locally into our production application very clearly shows when there is an invalid widgetId for a builtin widget. Examples:

This catches these errors before they happen (number is not a valid widgetId)

Todos

• Implemented feature
• Feature with pending implementation

Screenshots (if appropriate):

[whitelisab]

Thanks for the contribution @nib-bturner! This is a helpful DX improvement when working with the builtin widgetIds. Approving and merging! ⭐