Aligned the OneOf implementation with the specification

[glen-84]

Summary of the changes (Less than 80 chars)

• Aligned the OneOf implementation with the specification.

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• Updated the OneOf directive description.
• Renamed Oneof/oneof to OneOf.
• Renamed __Type.oneOf to __Type.isOneOf.
• Added test for { a: null }.
• Updated tests.
• (WIP) Moved validation from OneOf-specific rule to VariablesInAllowedPositions.

❓ Questions:

• How should we handle this change? (see Slack)
• "The @oneOf directive must not be provided by an Input Object type extension."
  • How should we handle this? Is this only for "real" extensions? Do we even support that? Does it apply to HC type extensions?
• ✅ I'm not sure how to implement VariableVisitor.IsNonNullPosition based on the spec.
  • Note: 3 tests currently skipped.
• Do we need to change something here?
• ✅ Should we add it as a built-in directive to all "BuiltIns"-like classes? (ref)
• Should we "unflag" it (remove config, on by default)? (seems to be the case in JS/Ruby and maybe Java).
  • Opened Enable OneOf support by default #8358.