fix(plugin-vue): hmr got error by transformSFC to inject code (fix #301)

[brenner8023]

Description

FIXED: #301

Some vite plugins like unocss:vue-scoped needs to rewrite hmrContext.read to inject their special code into SFC when hmr.

vite-plugin-vue uses fs.readFileSync to get the raw code, but the raw code doesn't have injected code by other plugins.

This PR allows vite-plugin-vue to get transformCode api from other plugins, and then replace fs.readFileSync with readCode function.

Additional context

• hmr error reproduction: https://stackblitz.com/edit/vitejs-vite-28qyca?file=src%2FApp.vue
  • Wait to start and change class text-blue to text-red
• playground for this PR: https://stackblitz.com/edit/vitejs-vite-6tuitn?file=src%2FApp.vue
  • u can run it locally

---

What is the purpose of this pull request?

• [X] Bug fix
• [ ] New Feature
• [ ] Documentation update
• [ ] Other

Before submitting the PR, please make sure you do the following

• [x] Read the Contributing Guidelines.
• [x] Read the Pull Request Guidelines and follow the PR Title Convention.
• [x] Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• [x] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• [x] Ideally, include relevant tests that fail without this PR but pass with it.