Support flat config and upgrade eslint to v9

[gracepark]

• closes https://github.com/github/web-systems/issues/2568, https://github.com/github/web-systems/issues/2569, Add support for flat config #513

This allows us to support flat config and provides testing for both:

• Upgrades the plugin to v9
• Flat config
• Legacy config remains in the same location
• I've created test-examples which allows testing for both legacy and flat configs:
  • For legacy, run: npm run test-examples:legacy on 8.57 without flat config
  • For flat, run: npm run test-examples:flat on 8.57 with flat config

Testing

I used yalc to test this package with other repos:

• clipboard-copy-element: This is on v8.42.0 and on legacy eslintrc.json config. I was able to "update" the eslint-plugin-github plugin and ran npm run lint and it ran as expected.
• codeql-action: eslint is on 8.57.1 and on flat config. I was able to "update" the eslint-plugin-github, ran npm run lint and it ran as expected.

[manuelpuyol]

This looks great! I love the new test examples and that you tested different repos 😄