Improve "expected error" types

[chriszarate]

• In special circumstances where we are faced with insufficient library types that we cannot augment, we should still work to preserve type safety. Instead of ignoring type errors and effectively relaxing to an any type, we can cordon the problematic entity and reexport it with strict typing. In this PR, we do this for registerBlockBindingsSource.
• Miscellaneous fixes:
  • Remove redundant export keywords in type declarations.
  • Move NullableKeys utility type to its own file.

[chriszarate]

this probably isn't right, but not sure what it should be. we can improve these once we start to use them

[alecgeatches]

Great solution to the problem for now! I like that you've isolated the type-ignores to a file and wrapped with better types.

[maxschmeling]

This is a good solution for now. I'd like for us to also try to get these types into the DefinitelyTyped repo so we don't have to have it in our codebase.