This change adds the indexed field for the characters in the definition
of sequential string types, and introduces support for recognizing the
various specific string types in v8_debug_helper. In an attempt to
avoid duplicating info about string instance types, it also refactors
String::Get so that StringShape (a simple class usable by postmortem
tools) can dispatch using a class that defines behaviors for each
concrete type.

Bug: v8:9376
Change-Id: Id0653040f6decddc004c73f8fe93d2187828c2c6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1735795
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63352}