feat: support platform

[fi3ework]

Summary

• add platform config
• auto externalize node built-in modules when platform is node

Concern of introducing platform config

• If we use keep using target field, some config will be changed internally (for now, externalsTypes for built-in modules has changed), which will lead to misalignment between Rsbuild and Rslib.
• If we using another field, such as platform as of now. output.target will conflict with it, we should drop it from Rsbuild available config for Rslib.

Related Links

Checklist

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

[fi3ework]

Using platform as a temp solution, it's set to be changed in the future once we made the final config decision.