GraphQL v15 support

[ardatan]

• Do not extend if the node is already extended (GraphQL v15 does that internally)
• Do not use introspectionQuery which is deprecated and removed in v15
• Fix asyncIterator typing

[yaacovCR]

See #32

Do you have any suggestions for supporting both v14 and v15. I imagine I will need to release a major version change (I am up to v8.5, so v9?) with the changes from #32 and then would have to backport any fixes to a V8 branch.

[ardatan]

I think those changes already support both v14 and v15. They are not only for v15.
getIntrospectionQuery is already introduced in v14, and changes about extensions check if extensions are included in actual astNode not to break anything on v14.

[yaacovCR]

I think because there are breaking changes anyway between v14 and v15 that it is better/cleaner to fix extensions issues as in a3aadcf rather than with check for existence of extension nodes after buildSchema. Do you disagree?

[yaacovCR]

See also #40

[ardatan]

I think those changes can be applied for both v14 and v15. I think supporting both versions would be better than having different releases for each. @yaacovCR

[yaacovCR]

This is addressed in graphql-tools-fork v8.6.0 which supports:
graphql v14.2.0 and above
graphql v15.0.0-rc2 and above