ESLint v9 support

[cexbrayat]

ESLint v9 has been released and we'll need to change a few things to properly support it.

npm run lint currently fails to run with eslint v9:

❯ npm run lint

> eslint-with-prettier@0.0.0 lint
> eslint . --ext .vue,.js,.jsx,.cjs,.mjs --fix --ignore-path .gitignore

Invalid option '--ext' - perhaps you meant '-c'?
You're using eslint.config.js, some command line flags are no longer available. Please see https://eslint.org/docs/latest/use/command-line-interface for details.

We'll also probably need to migrate the configuration file to use the flat config format:

• https://eslint.org/docs/latest/use/configure/migration-guide

[JoostKersjes]

See #451 for supporting the new default "Flat Config Files".

[jez9999]

I'd definitely like to see the project scaffolds using the flat config format ASAP. It greatly reduces the complexity of configuring ESLint, as well as setting a bunch of reasonable defaults the old system didn't and organizing things better.