fix: check for compiler.webpack existence

It's only available since webpack 5.1

Author: haoqunjiang

lib/plugin-webpack5.js

@@ -31,7 +31,9 @@ const ruleSetCompiler = new RuleSetCompiler([
 
 class VueLoaderPlugin {
   apply (compiler) {
-    const normalModule = compiler.webpack.NormalModule || require("webpack/lib/NormalModule")
+    const normalModule = compiler.webpack
+      ? compiler.webpack.NormalModule
+      : require('webpack/lib/NormalModule')
     // add NS marker so that the loader can detect and report missing plugin
     compiler.hooks.compilation.tap(id, compilation => {
       const normalModuleLoader = normalModule.getCompilationHooks(compilation).loader