Extends TypeDefs from TypeSource and not only String

[cdiaz]

I want to be able to use the @graphql-tools to load definitions from files, or code, or just be able to merge multiple definitions and pass them as, Source, DocumentNode, GraphQLSchema, DefinitionNode etc.
Accepting only string limits us

elysia-graphql-yoga/src/index.ts

Line 91 in f47ec0e

const TypeDefs extends string,