types(v-node-directive): changed expression type of VNodeDerective to optional string

fix vuejs#10871

Author: kawokas

types/vnode.d.ts

@@ -69,7 +69,7 @@ export interface VNodeDirective {
   name: string;
   value?: any;
   oldValue?: any;
-  expression?: any;
+  expression?: string;
   arg?: string;
   oldArg?: string;
   modifiers?: { [key: string]: boolean };