Make `typeResolver` optional in `nodeDefinitions`

[mhart]

AFAICT graphql-js allows you to leave out resolveType here:

https://github.com/graphql/graphql-js/blob/a149b89a3047f38750eaade1f220eb1317eb9217/src/type/definition.js#L684-L685

In which case it will fall back to using getTypeOf/isTypeOf

The problem is, nodeDefinitions doesn't seem to allow you to leave out the typeResolver function:

graphql-relay-js/src/node/node.js

Line 55 in 3dd1fc3

resolveType: typeResolver

Making this optional would certainly reduce a lot of boilerplate code

[mhart]

(FWIW, this is more of a Flow/type issue, not an in-practice issue)

[dschafer]

Yep, good call, we should do this.

[mhart]

Awesome, thanks 👍