fix: change OperationTypeNode to enum

Changes OperationTypeNode to enum instead of a union type.
This allows developers to use the type as `OperationTypeNode.QUERY`
instead of risking to msispell the string 'query'.

Example:

Instead of having to write

`operation === 'query'`

you can instead write

`operation === OperationTypeNode.QUERY`

src/language/ast.ts

@@ -364,7 +364,11 @@ export interface OperationDefinitionNode {
   readonly selectionSet: SelectionSetNode;
 }
 
-export type OperationTypeNode = 'query' | 'mutation' | 'subscription';
+export enum OperationTypeNode {
+  QUERY = 'query',
+  MUTATION = 'mutation',
+  SUBSCRIPTION = 'subscription'
+}
 
 export interface VariableDefinitionNode {
   readonly kind: 'VariableDefinition';