lib: use class fields in classes that revert to symbol properties due to missing snapshot support

https://bugs.chromium.org/p/v8/issues/detail?id=10704 is already fixed, so switch back to class fields instead of using symbol properties.

Side note: class field initialization performance has been optimized since v8 9.7 so this shouldn't cause performance regressions either.

Refs: https://github.com/nodejs/node/commit/b1c3909bd766327a569c2e4279a4670454f3f9db