Currently, the private name symbols are displayed in the block
scopes in DevTools, though these are just implementation details
of private fields. This patch hides them from the block scope
by marking variables with names starting with `#` as synthetic.

The private fields are still going to show up in the previews
of objects, only the key symbols themselves are going to be hidden.

Bug: v8:8773, chromium:982267
Change-Id: I059472d05c26a1f035ab92718a1b7e5ecafa8dc4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1741846Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Cr-Commit-Position: refs/heads/master@{#63112}