refactor: @cypress/webpack-preprocessor should be a peer dep

Author: haoqunjiang

packages/@vue/cli-plugin-e2e-cypress/__tests__/cypressPlugin.spec.js

@@ -13,6 +13,9 @@ if (!process.env.APPVEYOR) {
       }
     })
 
+    const pkg = JSON.parse(await project.read('package.json'))
+    expect(pkg.devDependencies).toHaveProperty('@cypress/webpack-preprocessor')
+
     const config = JSON.parse(await project.read('cypress.json'))
     config.video = false
     await project.write('cypress.json', JSON.stringify(config))

packages/@vue/cli-plugin-e2e-cypress/generator/index.js

@@ -5,6 +5,9 @@ module.exports = api => {
   })
 
   api.extendPackage({
+    devDependencies: {
+      '@cypress/webpack-preprocessor': '^3.0.0'
+    },
     scripts: {
       'test:e2e': 'vue-cli-service test:e2e'
     }

packages/@vue/cli-plugin-e2e-cypress/package.json

@@ -22,9 +22,11 @@
     "access": "public"
   },
   "dependencies": {
-    "@cypress/webpack-preprocessor": "^3.0.0",
     "@vue/cli-shared-utils": "^3.0.4",
     "cypress": "^3.0.2",
     "eslint-plugin-cypress": "^2.0.1"
+  },
+  "peerDependencies": {
+    "@cypress/webpack-preprocessor": "^3.0.0"
   }
 }