javascript: Add support for the `let` keyword

`let` is not yet part of the current ECMAScript standard but is part of
the ECMAScript 6 draft and is supported by Mozilla, and people already
use it in some contexts.

Also, the current ECMAScript standard marks `let` as a "considered
reserved word" (7.6.1.2), so it is already a reserved keyword in strict
mode.

See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let