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

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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