feat!: update to eslint v9, flat config only

[favna]

Please describe the changes this PR makes and why it should be merged:

This updates this repo to ESLint v9 and drops support for the legacy config style, only including the flat config style which is now the only supported config style for ESLint. This PR should be considered a breaking change! (v1.0.0 or v0.2.0, whichever you prefer)

Some of the plugins that this config requires are not yet updated for ESLint v9, the list can be tracked here: eslint/eslint#18093. I also marked some of them in the code with TODO [V9 UPDATE]. I aimed to ensure that at least the source code of this config itself can be linted with the current dependency versions.

Regarding the swap from eslint-plugin-i to eslint-plugin-import-x see: un-ts/eslint-plugin-import-x#29 (comment)

I also dropped all the js files in the root in favour of Node export mapping because ESLint now has a minimum Node version of 18.18.0 so we can safely use export mapping and declutter the repository a LOT.

Lastly I also removed all the husky, is-ci and pinst dependencies because no husky scripts were set up, so it was just unnecessary bloat.

Breaking change blurb for when doing a squash-and-merge:

BREAKING CHANGE: This package now only exports flat config. For more information see the [eslint migration guide](https://eslint.org/blog/2024/04/eslint-v9.0.0-released) and [flat config guide](https://eslint.org/blog/2022/08/new-config-system-part-2/). The README of this package has instructions for how to use this package with the new flat config system.