fix: improve alias merging

[antfu]

Description

Changes in #6344 expose the problem of alias merging strategy.

E.g.

mergeConfig(
  {
    resolve: { alias: { foo: 'old' }}
  },
  {
    resolve: { alias: { foo: 'new' }}
  }
)

With will be normalized to

[
  { find: 'foo', replacement: 'old' },
  { find: 'foo', replacement: 'new' }
]

Since @rollup/plugin-alias resolve the alias from top-down, it makes foo always resolved to foo, and makes the later config fail to override the previous.

Leading the error in Nuxt: nuxt/framework#2716

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
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• Ideally, include relevant tests that fail without this PR but pass with it.

[patak-dev]

The order here isn't important?

[antfu]

B should override A, which is the behavior of object merging, but not array previously