split Executor class into separate file

note that the Parser class is contained in lowercase parser.ts

src/execution/__tests__/executor-test.ts

@@ -18,6 +18,7 @@ import {
   GraphQLUnionType,
 } from '../../type/definition';
 
+import { Executor } from '../executor';
 import { execute, executeSync } from '../execute';
 
 describe('Execute: Handles basic execution tasks', () => {

src/graphql.ts

@@ -14,6 +14,7 @@ import type {
 import type { GraphQLSchema } from './type/schema';
 import { validateSchema } from './type/validate';
 
+import type { Executor } from './execution/executor';
 import type { ExecutionResult } from './execution/execute';
 import { execute } from './execution/execute';