fix(typeDefs): allow Array<string | DocumentNode>

[chentsulin]

makeExecutableSchema accepts Array<DocumentNode | string>, so I think this should be allowed too.

[glasser]

Good catch. I've taken it one step farther by having us use the types from @graphql-tools/schema directly. (In general it's not great when the types of our arguments are tied directly to third-party package arguments, but in this case all we ever do with these options is pass them to that package, and you can avoid this codepath by passing your own schema anyway.)