feat: improve the default splitting cache groups

[chenjiahan]

Summary

Improve the default splitting cache groups:

• No need to set reuseExistingChunk: true, it is enabled by default.
• Remove @babel/runtime from lib-polyfill.js, because Rsbuild uses SWC by default.
• Add priority: 0 for cache groups created by forceSplitting. This is consistent with the default behavior, but we want the priority to be explicit.

Checklist

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

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