Support directives on directive definitions in GraphQL (#19171)

changelog_unreleased/graphql/19171.md

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+#### Support directives on directive definitions (#19171 by @YBJ0000)
+
+[Directives on directive definitions](https://github.com/graphql/graphql-js/pull/4521) (and `extend directive` extensions) are now supported.
+
+<!-- prettier-ignore -->
+```gql
+# Input
+directive @a @b on QUERY
+extend directive @a @b
+
+# Prettier stable
+Error: Syntax Error: Expected "on", found "@".
+
+# Prettier main
+directive @a @b on QUERY
+extend directive @a @b
+```