Migrate to eslint flat config

[int128]

git checkout -B migrate-eslint-config

pnpm remove @typescript-eslint/eslint-plugin @typescript-eslint/parser
pnpm add -D typescript-eslint
sed -i '' -e 's|eslint \*\*/\*.ts|eslint .|' package.json
cp ../typescript-action/eslint.config.js .
rm .eslintrc.*
pnpm lint

For CJS:

mv eslint.config.js eslint.config.mjs
pnpm lint

git add .
git commit -m 'Migrate to eslint flat config'
gh pr create -f --body '- https://github.com/int128/typescript-action/issues/780'