Add directive definition support to opt-in features RFC

[glen-84]

Summary

Updates the opt-in features RFC to support directives, now that directives on directive definitions are part of the draft spec.

• Add DIRECTIVE_DEFINITION to the @requiresOptIn locations (and the docstring), with an example marking a custom directive opt-in.
• List directive definitions alongside the other kinds (field, input field, argument, enum value) in the introspection prose, rather than adding a separate __Schema/__Directive block — matching how the section already handles arguments and enum values.
• Align the includeDeprecated introspection argument with the current draft spec (Boolean! instead of Boolean).
• Update the "marker directives" alternate solution: drop the now-resolved "requires a grammar change" con, and place the applied directive before the on clause to match the DirectiveDefinition grammar.
• Fix an "include" → "included" typo in the introspection prose.

[martinbonnin]

Thanks!

[martinbonnin]

Good catch 👍

[martinbonnin]

Nice!