Use ESM

[chenxsan]

The whole remark ecosystem has migrated to ESM now. Packages like node-fetch did it as well. Thus I think we should commence the migration, otherwise we might get stuck in the CommonJS world with a lot of outdated packages.

• Closes build(deps-dev): bump remark-emoji from 2.2.0 to 3.0.1 #5337.
• Closes build(deps-dev): bump node-fetch from 2.6.1 to 3.0.0 #5389
• Closes build(deps-dev): bump remark from 13.0.0 to 14.0.1 #5350
• Closes build(deps-dev): bump remark-html from 13.0.1 to 14.0.0 #5272
• closes build(deps-dev): bump remark-gfm from 1.0.0 to 2.0.0 #5280
• closes build(deps-dev): bump remark-autolink-headings from 6.1.0 to 7.0.1 #5328
• closes build(deps-dev): bump remark-slug from 6.1.0 to 7.0.0 #5278
• closes build(deps): bump unist-util-visit from 2.0.3 to 4.0.0 #5243

some problems

1. remark-gfm 2.0.0 is not compatible with @mdx-js/loader. While replacing @mdx-js/loader with xdm would fix the problem, migrating to ESM-only xdm is much difficult than I expected when we have server side rendering enabled, so I might do it in another pull request if I can figure out how.

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[snitin315]

Nice!