[graphql-js] separate execute/graphql functions that support AsyncGenerator responses

[robrichard]

Context

The current api changes to graphql-js to support defer/stream introduce a breaking change in the return type of the execute and graphql functions. The return type changes from returning a Promise or value, to returning either an AsyncGenerator, Promise, or value.

We previously implemented separate new functions that support the new return type, preserving the return type in the existing functions. However, this led to a lot of code duplication so this was abandoned.

Additional discussion: graphql/graphql-js#2848 (comment)