Support ESLint v9 and its default flat config
+ Flat config file eslint.config.mjs
+ Work with exports instead of main
+ no-fallthrough has a new reportUnusedFallthroughComment option that we turn on
+ Replace eslint-plugin-node with community-maintained eslint-plugin-n
+ Adapt the core rules that moved to @stylistic/eslint-plugin
* Push node requirement to latest LTS (v20.12.1) or later
* Update README.md to follow flat