Use ambiguous-object-type lint and exact-by-default syntax in index.js.flow

[jbrown215]

Now that some projects are using exact-by-default syntax, we should change the exported flow types here to be compatible with both exact-by-default and non-exact-by-default codebases.

To do that, we can use the ambiguous-object-type lint, which will error on uses of {} without explicitly being exact {||} or inexact {...}.

To test, I ran flow check and made sure there were no errors in index.js.flow, but there were other module resolution errors.

[jbrown215]

Ah, will need to use flow 0.113

[bcherny]

Thanks @jbrown215!