Migrate to flat config (eslint v9+)

[aorcsik]

Fix #12, #13 and #18

Disclaimer:

• This fix keeps functionality (almost) identical to the original, just adds eslint v9 and flat config support
• There is a rewrite of this plugin that provides eslint v9 support and handles import aliases too by @mbonaci, see: https://github.com/anza-xyz/eslint-plugin-require-extensions/issues/10#issuecomment-2103120190