moved @types/graphql from dependencies to dev-dependencies

[mechanical-turk]

As described in the FAQ page, multiple versions of @types/graphql causes build issues as the compiler gets confused. This is an attempt at decreasing the probability of having multiple versions of the package by moving @types/graphql to dev-dependencies.

[mechanical-turk]

Tests passing:

[MichalLytek]

Unfortunately, we can't do that. It would cause that without manual installing of @types/graphql your app won't even compile, return type from buildSchema will be any, etc.

We could only move it do peerDependency and it would have to be done together with main graphql dependency but that was rejected a few weeks ago - see #148 and #144.