tools: avoid let in for loops

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
PR-URL: https://github.com/nodejs/node/pull/9049


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>