fix(plugin-vue): deprecate inline main (fix #31)

[aweikalee]

Description

vite-plugin-vue should deprecate inline main, because .vue file can't be processed as js/ts.

fix: #31, vite#13756

Additional context

When using vite dev, lang="js" or lang="ts" will be skip build,

Live demo: stackblitz

lang="js" or lang="ts" will as inline main, skip js/ts build. Because its extname is .vue.

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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

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