tools: decrease code duplication for isString() in lint rules

This commit makes isString() a reusable utility
function for core's custom ESLint rules.

PR-URL: https://github.com/nodejs/node/pull/27719


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>