v8::String::IsExternal is confusing since it only checks for external
two byte strings. The goal is to reintroduce String::IsExternal which
checks for one and two byte external strings after removing the old,
misleading api method.

- Add String::IsExternalTwoByte
- Deprecate String::IsExternal for now since it is misleading


Bug: v8:10641
Change-Id: I8989de7576c823846e0536fc1898e769b6d68c87
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2284495
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69674}