fix: fix compatibility with webpack 3

According to the documentation
https://vue-loader.vuejs.org/guide/extract-css.html#webpack-3
and some private feedbacks, there are still many users use
vue-loader v15 along with webpack 3.

Author: haoqunjiang

lib/plugin.js

@@ -1,7 +1,7 @@
 const webpack = require('webpack')
 let VueLoaderPlugin = null
 
-if (webpack.version[0] > 4) {
+if (webpack.version && webpack.version[0] > 4) {
   // webpack5 and upper
   VueLoaderPlugin = require('./plugin-webpack5')
 } else {

package.json

@@ -34,7 +34,7 @@
   },
   "peerDependencies": {
     "css-loader": "*",
-    "webpack": "^4.1.0 || ^5.0.0-0"
+    "webpack": "^3.0.0 || ^4.1.0 || ^5.0.0-0"
   },
   "peerDependenciesMeta": {
     "cache-loader": {