Build: migrate eslint config to flat config

See https://eslint.org/docs/latest/use/configure/configuration-files-new

One rule currently blocked by
import-js/eslint-plugin-import#2556

Author: timmywil

.eslintignore

@@ -1,5 +1,5 @@
 #!/usr/bin/env sh
 . "$(dirname -- "$0")/_/husky.sh"
 
-npm run lint:cache
+npm run lint
 npx grunt qunit_fixture