fix(plugin-vue): don't use object spread in the config hook

[haoqunjiang]

Description

The config object will be merged recursively in vite core, so object
spread shouldn't be used:

vite/packages/vite/src/node/config.ts

Line 308 in 5cf4e69

config = mergeConfig(config, res)

Otherwise, it would cause an array field to be concatenated with itself.

Fixes #5150

Additional context

I didn't add a test for that issue, because it's not a use case that we intended to support. The documentation clearly stated that the define field is of type Record<string, string>.

But it's a good programming practice to remove the unnecessary object spread anyway.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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