Support eslint flat config

[silverwind]

Likely depends on import-js#2556 but it's good to track nonetheless.

Ref: eslint/eslint#18093

[JounQin]

eslint-plugin-i is now eslint-plugin-import-x.

I'm focusing on TypeScript migration first, see also #41.

After that, I'll start working on new features like supporting flat config.

[meteorlxy]

Just to mention, some rules will break when using flat config. See this comment

In brief, eslint will fail to use eslint.config.js as config file after enabling import/no-unused-modules with unusedExports: true

Related issues:

• Bug: can not find flat file config eslint/eslint#16485
• import/no-unused-modules breaks --no-eslintrc and eslint fails import-js/eslint-plugin-import#2907
• Add note(s) to docs about flat configs and/or no-unused-modules' unusedExports not being supported in flat configs import-js/eslint-plugin-import#2964