fix(plugin-react): only detect preamble in hmr context

[antfu]

Description

In Vitest, we do client transform for components to run on node side. I think it make sense for the plugin to only check preamble when hmr is available

vitest-dev/vitest#125

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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