src: add helpers for creating cppgc-managed wrappers

This patch adds helpers for wrapper classes based on cppgc (Oilpan)
in `src/cppgc_helpers.h`, including `node::CppgcMixin` and
`ASSIGN_OR_RETURN_UNWRAP_CPPGC`, which are designed to have
similar interface to BaseObject helpers to help migration.
They are documented in the `CppgcMixin` section in `src/README.md`

To disambiguate, the global `node::Unwrap<>` has now been moved
as `node::BaseObject::Unwrap<>`, and `node::Cppgc::Unwrap<>`
implements a similar unwrapping mechanism for cppgc-managed
wrappers.

PR-URL: https://github.com/nodejs/node/pull/52295
Refs: https://github.com/nodejs/node/issues/40786
Refs: https://docs.google.com/document/d/1ny2Qz_EsUnXGKJRGxoA-FXIE2xpLgaMAN6jD7eAkqFQ/edit
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>

node.gyp

@@ -202,6 +202,7 @@
       'src/compile_cache.h',
       'src/connect_wrap.h',
       'src/connection_wrap.h',
+      'src/cppgc_helpers.h',
       'src/dataqueue/queue.h',
       'src/debug_utils.h',
       'src/debug_utils-inl.h',