tools: update ESLint to 6.4.0

PR-URL: https://github.com/nodejs/node/pull/29553
Refs: https://github.com/eslint/eslint/releases/tag/v6.4.0


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>