fix: types and docs of umd external object

[LingyuCoder]

Summary

relate to #8248

Cause the external object value can only be used when libraryTarget or externalTarget is umd, and if it will throw error when commonjs or commonjs2 or root is not set. So we can use zod to validate the external object value.

The amd field is optional because it can generate define([]) when not set, but there is a bug here in rspack that generated define([null]). This pr also fix it.

Not close the issue because when libraryTarget is not umd, the external object value is also valid but will generate a wired code. For example, libraryTarget: "var" and react: { root: "React", commonjs: "react" } will generate var external_react_root_react_commonjs_react = undefined which can not actually work.

So we may need a better validation here to check libraryTarget and externals.

Checklist

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

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