Add support for directives on directive definitions

[BoD]

Allow directives on directive definitions, based on this spec PR which introduces this syntax:

directive @onDirective on DIRECTIVE_DEFINITION

directive @foo @onDirective on OBJECT

directive @baz @deprecated(reason: "...") on OBJECT

extend directive @quux @deprecated(reason: "...")

Disclaimer

First time on this codebase and am also not a JS/TS person, so obvious mistakes and/or missing pieces are very likely 😅 Any help and feedback to improve this PR are very welcome 🙏. The tests seem to pass but additional tests may be needed.

[vercel]

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