spec: Directives on variable definitions

[saihaj]

as per spec https://spec.graphql.org/draft/#ExecutableDirectiveLocation directives can exist on VARIABLE_DEFINITION need to update this.

graphql-parser/src/schema/ast.rs

Lines 430 to 453 in b322a14

	impl DirectiveLocation {
	/// Returns GraphQL syntax compatible name of the directive
	pub fn as_str(&self) -> &'static str {
	use self::DirectiveLocation::*;
	match *self {
	Query => "QUERY",
	Mutation => "MUTATION",
	Subscription => "SUBSCRIPTION",
	Field => "FIELD",
	FragmentDefinition => "FRAGMENT_DEFINITION",
	FragmentSpread => "FRAGMENT_SPREAD",
	InlineFragment => "INLINE_FRAGMENT",
	Schema => "SCHEMA",
	Scalar => "SCALAR",
	Object => "OBJECT",
	FieldDefinition => "FIELD_DEFINITION",
	ArgumentDefinition => "ARGUMENT_DEFINITION",
	Interface => "INTERFACE",
	Union => "UNION",
	Enum => "ENUM",
	EnumValue => "ENUM_VALUE",
	InputObject => "INPUT_OBJECT",
	InputFieldDefinition => "INPUT_FIELD_DEFINITION",