fix: allow _ in watch paths (element compat)

Author: yyx990803

src/core/util/lang.js

@@ -30,7 +30,7 @@ export function def (obj: Object, key: string, val: any, enumerable?: boolean) {
 /**
  * Parse simple path.
  */
-const bailRE = new RegExp(`[^${unicodeLetters}.$]`)
+const bailRE = new RegExp(`[^${unicodeLetters}.$_]`)
 export function parsePath (path: string): any {
   if (bailRE.test(path)) {
     return