Convert to ESLint 8 New Flat Config, Support for ESLint 9

[karlhorky]

ESLint 8 has a new "flat" configuration format:

https://eslint.org/docs/latest/user-guide/configuring/configuration-files-new

Currently with ESLint 9, using eslint-plugin-upleveled results in the following error:

ESLint: 9.3.0

TypeError: context.getScope is not a function
Occurred while linting ...
Rule: "upleveled/no-submit-handler-without-preventDefault"

Seems like eslint-plugin-react merged and released a PR adding compatibility for the new Flat Config format here, which could prove to be a helpful guide how we can implement here too:

• [New] support new config system jsx-eslint/eslint-plugin-react#3429
• https://github.com/jsx-eslint/eslint-plugin-react/releases/tag/v7.32.0

Once we have support in this plugin, then we can also switch to the new Flat Config in @upleveled/eslint-config-upleveled: upleveled/eslint-config-upleveled#160

[karlhorky]

Worked around this temporarily in:

• Release: eslint-config-upleveled@8.1.7
• PR: Fixup rules from eslint-plugin-upleveled eslint-config-upleveled#378