fix: only handle merge ssr.noExternal

[bluwy]

Description

Previously noExternal in any level of the config would have special true handling. Now this is only handled for ssr.noExternal.

Additional context

Found this while discussing the potential use of mergeConfig in vite-plugin-svelte

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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