fix(cli): load @rspack/dev-server on demand

[xc2]

Summary

fixes #6359

stop loading @rspack/dev-server on rspack build to prevent process.env.WEBPACK_SERVE being set to true on cli's build mode.

Example

// rspack.config.js
console.log(process.env.WEBPACK_SERVE);
module.exports = {
  // ...
};

command	BEFORE	AFTER
rspack build	"true"	undefined
rspack serve	"true"	"true"

Performance

requiring @rspack/dev-server will resolve and load 433 files, so this change will also make the cli bootstrap faster on build mode.

When testing on mac studio m2 max:

• ~100ms faster (io preheated)

Checklist

• Tests updated (or not required).
• Documentation updated (or not required) - no eed

[netlify]

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[JSerFeng]

Thanks !