feat!: replace output.targets with output.target

[9aoy]

Summary

• breaking change: Replace output.targets with output.target
• chore: Generate rspack / webpack config based on environment instead of output.targets

Each environment has and only corresponds to one target. Multiple environments can have the same target.

one environment = one rsbuild configuration = one rspack configuration = one build outputs.

Related Links

#2620

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[chenjiahan]

Please add to: #2508