src: make `CompiledFnEntry` a `BaseObject`

In particular:

- Move the class definition to the relevant header file,
  i.e. `node_contextify.h`.
- Make sure that class instances are destroyed on
  `Environment` teardown.
- Make instances of the key object traceable in heap dumps. This is
  particularly relevant here because our C++ script → map key mapping
  could introduce memory leaks when the import function metadata refers
  back to the script in some way.

Refs: https://github.com/nodejs/node/pull/28671

PR-URL: https://github.com/nodejs/node/pull/28782


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Rich Trott <rtrott@gmail.com>