perf: remove node-fetch polyfill

[chenjiahan]

Summary

Remove node-fetch polyfill to reduce 7.45MB install size:

• In Node.js >= 18, fetch is builtin and enabled by default.
• In Node.js 16, fetch can be enabled by the --experimental-fetch flag, I have updated the bin file to enable it.
• In Rspress 2.0 we can remove this flag from the bin file.

Other Change

This PR also removed mdast-util-mdxjs-esm from two packages because they never use it:

TODO

I think the remote search should be redesigned in Rspress 2.0 as it is now a ByteDance SCM only feature.

See:

rspress/packages/core/src/node/searchIndex.ts

Line 42 in a373b32

if (isProduction() && isSCM() && config?.search?.mode === 'remote') {

Checklist

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

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