fix: add getVueMajor util

packages/@vue/cli-service/lib/util/getVueMajor.js

@@ -0,0 +1,13 @@
+const { semver, loadModule } = require('@vue/cli-shared-utils')
+
+/**
+ * Get the major Vue version that the user project uses
+ * @param {string} cwd the user project root
+ * @returns {2|3}
+ */
+module.exports = function getVueMajor (cwd) {
+  const vue = loadModule('vue', cwd)
+  // TODO: make Vue 3 the default version
+  const vueMajor = vue ? semver.major(vue.version) : 2
+  return vueMajor
+}