frontend: migrate to flat config

Using npx @eslint/migrate-config .eslintrc.json

Then add the additional packages needed
npm install -D -E globals @eslint/js @eslint/eslintrc @eslint/compat

Add paths from the .gitignore file to the ignored patterns.
Add data, dist and public/twemoji explicitly because that did not work with the
includeIgnoreFile.
Remove the options from the call to eslint which are not
supported anymore in the package.json.

Issue: ecamp#5282