Add support for ESLint 9

[Jacob Carpenter]

ESLint 9 is released, but it looks like eslint-plugin-jsx-a11y does not yet support it.

There are breaking API changes: https://eslint.org/docs/latest/use/migrate-to-9.0.0#breaking-changes-for-plugin-developers

There is also a new default config format ("flat config") that probably also needs some adjustment in the exported recommended rules; though, support for flat config could be considered a separate issue.

[Jordan Harband]

Yes, that's right.

In eslint 9, RuleTester only supports flat config, so it makes things more complicated.

I think we need flat config support before we have eslint 9 support.

[Jordan Harband]

See #891.

[Hyeonjun (Jello)]

I'm using the plugin with the flat config.
Hope it fully supports eslint flat config asap! 👍🏻

import pluginJsxA11y from 'eslint-plugin-jsx-a11y';

export default [
  ...
  {
    plugins: {
      'jsx-a11y': pluginJsxA11y,
    },
    rules: pluginJsxA11y.configs.recommended.rules,
  },
  ...
];