vm: migrate ContextifyScript to cppgc

[joyeecheung]

This PR includes two commits. One adds a helper mixin node::CppgcMixin to src/cppgc_helpers.h along with some documentation in src/README.md to facilitate migration from BaseObject to cppgc-based memory management (Oilpan), and another migrates ContextifyScript to cppgc which will be the first wrapper class in Node.js to use it. This class is chosen because it doesn't have any externally managed data - most other wrapper classes do and they will need to wait for https://chromium-review.googlesource.com/c/v8/v8/+/5630497.

For more information about migrating to Oilpan (cppgc) in Node.js, see the design doc and Chromium's documentation on Oilpan

Refs: https://github.com/nodejs/node/issues/40786 Refs: https://docs.google.com/document/d/1ny2Qz_EsUnXGKJRGxoA-FXIE2xpLgaMAN6jD7eAkqFQ/edit

Local benchmark numbers show a small improvement in compiling small scripts, likely due to improved GC performance:

                                                                                                                                    confidence improvement accuracy (*)   (**)  (***)
vm/compile-script-in-isolate-cache.js n=1000 filename='test/fixtures/snapshot/typescript.js' type='with-dynamic-import-callback'                    0.37 %       ±0.51% ±0.69% ±0.91%
vm/compile-script-in-isolate-cache.js n=1000 filename='test/fixtures/snapshot/typescript.js' type='without-dynamic-import-callback'          *      1.01 %       ±0.87% ±1.18% ±1.56%
vm/compile-script-in-isolate-cache.js n=1000 filename='test/fixtures/syntax/good_syntax.js' type='with-dynamic-import-callback'            ***      6.45 %       ±1.39% ±1.86% ±2.44%
vm/compile-script-in-isolate-cache.js n=1000 filename='test/fixtures/syntax/good_syntax.js' type='without-dynamic-import-callback'         ***      8.69 %       ±0.99% ±1.32% ±1.72%