Revert "Revert "[parser] Implements proposal-numeric-separator.""

This reverts commit 782f6401.

Original CL is https://chromium-review.googlesource.com/c/v8/v8/+/923441

Bug: v8:7317
Change-Id: I6f541c038bad0cff625094ba84aebe582bdeb12f
Reviewed-on: https://chromium-review.googlesource.com/945034Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51749}