This is mostly an auto-conversion done by several tools.

- use let / const
- use arrow functions
- use template strings

There are some additional manual rewrite required to modernize the
code further.

Change-Id: I63a7a43b05b14b33ad9941350d3d5f26aab10ba0
Bug: v8:10667
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2519564Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71080}