feat: Warn on deprecated enum values used as arguments

[jungwoo3490]

Summary

GraphQL allows @deprecated on enum values, but Relay wasn't warning when a deprecated enum value was used as a field or directive argument. This PR adds that validation alongside the existing deprecated field/argument checks.

enum Status {
  ACTIVE
  LEGACY @deprecated(reason: "Use ACTIVE instead.")
}

# Before: no warning
# After: The enum value `LEGACY` of type `Status` is deprecated. Deprecation reason: "Use ACTIVE instead."
fragment Foo on Query {
  user(status: LEGACY)
}

The warning message wording follows the existing patterns in the codebase. Open to suggestions if there's a better way to phrase it!

Note

Variable values ($status) and deeply nested enum values inside constant objects/arrays are not yet covered. Left a TODO comment in the code.