feat: temporarily fix source map by patching babel

Author: yyx990803

packages/@vue/cli-plugin-babel/index.js

@@ -1,3 +1,8 @@
+// hack babel's codegen to fix source map.
+// this is a temporary patch before the actual change is released.
+// TODO remove after upgrading Babel to 7.0.0-beta.47
+require('./patchBabel')
+
 module.exports = (api, {
   parallel,
   transpileDependencies