tools: avoid let in for loops
Checklist
-
tests and/or benchmarks are included -
commit message follows commit guidelines
Affected core subsystem(s)
Tools.
Description of change
This adds a new ESLint tool to check for let declarations within the for, forIn, forOf expressions.
Fixes: https://github.com/nodejs/node/issues/9045 Ref: https://github.com/nodejs/node/pull/8873