Allow field names starting with a leading `_`

[pavelnikolov]

The 2021 GraphQL spec allows (field) names starting with a leading _. However, in this library it is not possible to have a resolver field/method in Go starting with a leading _ because it would be private and therefore not visible from other packages.

Done when:

type Query {
    _hello: String!
}

is supported and the library can resolve a query like this:

query {
   _hello
}