feat: Adds validateSchema

Author: NeedleInAJayStack

Sources/GraphQL/Language/AST.swift

@@ -360,7 +360,7 @@ public func == (lhs: Definition, rhs: Definition) -> Bool {
     return false
 }
 
-public enum OperationType: String {
+public enum OperationType: String, CaseIterable {
     case query
     case mutation
     case subscription

Sources/GraphQL/Type/Schema.swift

@@ -34,7 +34,7 @@ public final class GraphQLSchema {
     let extensionASTNodes: [SchemaExtensionDefinition]
 
     // Used as a cache for validateSchema().
-    let validationErrors: [GraphQLError]?
+    var validationErrors: [GraphQLError]?
 
     public let queryType: GraphQLObjectType?
     public let mutationType: GraphQLObjectType?