This moves every single class/function out of include/v8.h into a
separate header in include/, which v8.h then includes so that
externally nothing appears to have changed.

Every include of v8.h from inside v8 has been changed to a more
fine-grained include.

Previously inline functions defined at the bottom of v8.h would call
private non-inline functions in the V8 class. Since that class is now
in v8-initialization.h and is rarely included (as that would create
dependency cycles), this is not possible and so those methods have been
moved out of the V8 class into the namespace v8::api_internal.

None of the previous files in include/ now #include v8.h, which means
if embedders were relying on this transitive dependency then it will
give compile failures.

v8-inspector.h does depend on v8-scripts.h for the time being to ensure
that Chrome continue to compile but that change will be reverted once
those transitive #includes in chrome are changed to include it directly.

Full design:
https://docs.google.com/document/d/1rTD--I8hCAr-Rho1WTumZzFKaDpEp0IJ8ejZtk4nJdA/edit?usp=sharing

Bug: v8:11965
Change-Id: I53b84b29581632710edc80eb11f819c2097a2877
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3097448Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76424}