Type definitions for the import from `@types/braintree-web-drop-in` not available

[adrian-delgado]

This package doesn't expose the types from @types/braintree-web-drop-in, so for example the prop onInstance if of type (instance: any) => void.

This should be fixed easily by making @types/braintree-web-drop-in a normal dependency (instead of a devDependency).
(https://stackoverflow.com/a/46011417/12412337 provides more info about this).