Drop CJS (module.exports)

[JasonWeinzierl]

Are there any major objections to switching this plugin to ESM-only?

Currently, we distribute both ESM and CJS flavors in the same package. This means the plugin is ~300KB, twice the size it needs to be. We also need to maintain the build pipeline for supporting both ESM and CJS.

I'm proposing an ESM-only distribution, based on the following assumptions:

• eslint supports .mjs config files (export default), and users should prefer this config style instead of CJS.
• The ecosystem will continue to migrate toward ESM and drop CJS.
  • Node.js 18 is EOL, and Node.js >= 20.19.0 supports require(ESM), so we could bump node versions.
  • Maybe wait until tseslint or eslint drops Node.js 18 support and align with their upgrade? (Enhancement: Convert packages to ESM typescript-eslint/typescript-eslint#11379)
• We already don't support the old .eslintrc format, so dropping backward compatibility is ok.
• We still haven't released v1.0.0, so this is a great opportunity to introduce a breaking change before announcing stability.

Further info: https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c

[JasonWeinzierl]

Consider switching to tsdown instead of tsup when doing this.