Suggest remove main and module if have exports

[bluwy]

The exports field supersedes other mainFields, if the tooling supports the exports field.

exports is supported since:

• Node.js v12.7 (Release: 2019-07-23) (history)
  • Around the same time when ESM support is stabilized (history)
  • Earliest ESM support is v8.5, but adoption could be considered low as it was experimental until v12.20
• Webpack 5 (Release: 2020-10-10)
  • Webpack 4 does not support exports, which is what the majority still uses.
• @rollupjs/plugin-node-resolve v11.1 (Release: 2021-01-15)
• Parcel 2.9.0 (Release: 2023-05-26) - Use package.json#exports map parcel-bundler/parcel#4155
• Other recent tooling should support this by default.

exports is still not supported by:

• eslint-plugin-node - no-unresolved is not aware of exports definition in package.json import-js/eslint-plugin-import#1810 (new fork should be used instead: https://github.com/eslint-community/eslint-plugin-n)

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Given the state of support, perhaps we have to wait a bit longer. Because many packages will have both main/module and exports, and suggesting this would be a huge breaking change without much benefit.

I think there's now enough support to start providing this suggestion. There's still the concern that it's totally fine to keep using the main fields just in case, but I think this is a good nudge towards a single way to export things, and will help simplify library publishing in the future.

This suggestion is initially brought up in #21.