After deprecation and removal of the old misleading API we re-add
v8::String::IsExternal which returns true for both, external one-byte and
external two-byte strings.

Bug: v8:10641
Change-Id: I4c66d4df891f7180c7a727a45c1fbd254a7f5c02
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2726512Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73137}