feat: add new mode config

[chenjiahan]

Summary

In the past, Rsbuild internally relied too much on process.env.NODE_ENV, which resulted in coupling between the build mode and NODE_ENV.

This PR introduces a new mode config for Rsbuild. This config is similar to Rspack's mode config and can be used to specify the build mode of Rsbuild without needing to modify process.env.NODE_ENV.

Related Links

• https://rspack.dev/config/mode

Checklist

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

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