minify bundle size

[yokots]

以 hippy-vue-next-demo index.android.js 为例，优化前 328KB， 优化后 302 KB。项目越大效果越明显。

优化点：

1. 优化 hippy-vue-css-loader 生成代码体积。具体为将 HIPPY_VUE_STYLES 字符串抽取变量，减少重复。
2. 去掉 webpack 配置中的 webpack.NamedModulesPlugin , 此 Plugin 以废弃且生成的代码中会携带源码路径信息。

优化前生成代码

优化后生成代码

[hippy-service]

Hi, @yokots. Thanks for your PR! 👏

🏷️ You can leave a comment in this PR with #help tag when you need help (e.g. some status checks run failed due to internal issue), admin team members will help asap.

[tencent-adm]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[hippy-service]

After a quick scan, I have approved workflow to run.

• SHA: cd5c0f9

🏷️ New commits in this PR would not be tested automatically until this pull request is reviewed by our collaborators.
🏷️ No need to worry about the status of merge_guard and [gh] pull request merge guard / merge_guard (pull_request_target) checks, once this pull request is met merge requirements, it will be automatically converted to successful status.

[hippy-service]

Pay attention 🛎️ !!
There has been no activity on this PR for 2 months, so I will label it stalled.
It will be automatically closed in 30 days if no more activity. Feel free to leave a comment if you have any questions.

[hippy-service]

Sorry, closing this PR because it has stalled for over 4 months.
Feel free to reopen if this PR is still relevant, or to ping the collaborator if you have any questions.