GraphQLSchema does not completely validate names

The `validateName` function that is used to validate lots of schema parts is currently only checking that names are not reserved:
https://github.com/graphql/graphql-js/blob/6b253e7159a6a6c64c05bcc6bb863fef5a42eeb7/src/type/validate.ts#L206-L217
It should also check that the `name` complies with https://spec.graphql.org/October2021/#Name, e.g. by testing against the regex `/^(?!__)[A-Za-z_][A-Za-z0-9_]*$/`.
~Otherwise it's possible to construct schemas (via the constructor, not by parsing) that upon printing would lead to invalid syntax, or fields which could never be queried.~ (Not the case, see below)

Start in src/type/validate.ts at the validateName function linked in the issue, then compare its current checks with the GraphQL October 2021 Name specification. Verify that schema names violating the specified syntax are rejected, including names beginning with __, and add or update focused validation coverage if the surrounding code indicates where it belongs.

Written by the indexing model from the issue text.