[GraphQL] Add "getVisitFn" function

types/graphql/language/index.d.ts

@@ -6,4 +6,10 @@ export { createLexer, TokenKind, Lexer } from './lexer';
 export { parse, parseValue, parseType, ParseOptions } from './parser';
 export { print } from './printer';
 export { Source } from './source';
-export { visit, visitInParallel, visitWithTypeInfo, BREAK } from './visitor';
+export {
+    visit,
+    visitInParallel,
+    visitWithTypeInfo,
+    getVisitFn,
+    BREAK
+} from './visitor';

types/graphql/language/visitor.d.ts

@@ -47,3 +47,9 @@ export function visit(root: any, visitor: any, keyMap?: any): any;
 export function visitInParallel(visitors: any): any;
 
 export function visitWithTypeInfo(typeInfo: any, visitor: any): any;
+
+/**
+ * Given a visitor instance, if it is leaving or not, and a node kind, return
+ * the function the visitor runtime should call.
+ */
+export function getVisitFn(visitor: any, kind: any, isLeaving: any): any;