tools: enable eslint one-var rule

PR-URL: https://github.com/nodejs/node/pull/18831


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>