As Nico pointed out in [1], it is a little strange that the pair of
annotations "@export @customCppClass" behaves similarly to the keyword
"extern": both indicate that the class is defined in a C++ file and
Torque generates only a base class template for it. In this change, I
explore a possible alternative which might be more consistent.

Removed annotations:
- @customCppClass, which required @export, instructed Torque to only
  generate a base class template instead of a full class.
- @customMap, which also required @export, instructed Torque to not emit
  code for setting up a unique Map instance for the class.

Added annotations:
- @generateUniqueMap, which requires extern, instructs Torque to emit
  code for setting up a unique Map instance for the class.
- @generateFactoryFunction, which requires extern, instructs Torque to
  emit a function for creating a class instance.

Subtracting two annotations and adding two others still leaves us with
way too many annotations, but the usage of "extern" becomes more
consistent and I think that the new opt-in annotations might be easier
to understand.

[1] https://docs.google.com/document/d/1q_gZLnXd4bGnCx3IUfbln46K3bSs9UHBGasy9McQtHI/edit

Bug: v8:7793
Change-Id: Ic9e147a095bc492d6645001b9275357386e8adcf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3266008Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#77799}