fix: handle rule.use being a string (ref: #1256)

Author: yyx990803

lib/plugin.js

@@ -91,13 +91,15 @@ class VueLoaderPlugin {
       // otherwise RuleSet throws error if no option for a given ref is found.
       if (vueRule.loader || vueRule.loaders) {
         vueRule.options = { ident }
-      } else if (vueRule.use) {
+      } else if (Array.isArray(vueRule.use)) {
         const use = vueRule.use[vueLoaderUseIndex]
         if (typeof use === 'string') {
           vueRule.use[vueLoaderUseIndex] = { loader: use, options: { ident }}
         } else {
           use.options = { ident }
         }
+      } else if (typeof vueRule.use === 'string') {
+        vueRule.use = [{ loader: vueRule.use, options: { ident }}]
       } else {
         throw new Error(
           `VueLoaderPlugin Error: this should not happen. Please open an issue ` +