Improve type declarations

[andycodesstuff]

A rework of the existing type declarations to fix various errors:

1. The previous type declarations used field: null | ... instead of field?: ... to define optional fields
2. Using React.HTMLProps<HTMLButtonElement> on the TellerConnect component to provide better props IntelliSense
3. Rename onLoad method to onInit to match docs
4. Remove unnecessary | string and [key: string]: any from types
5. Remove unnecessary types