Previously when managing the importModuleDynamically callback of
vm.compileFunction(), we use an ID number as the host defined option
and maintain a per-Environment ID -> CompiledFnEntry map to retain
the top-level referrer function returned by vm.compileFunction() in
order to pass it back to the callback, but it would leak because with
how we used v8::Persistent to maintain this reference, V8 would not
be able to understand the cycle and would just think that the
CompiledFnEntry was supposed to live forever. We made an attempt
to make that reference known to V8 by making the CompiledFnEntry weak
and using a private symbol to make CompiledFnEntry strongly
references the top-level referrer function in
nodejs#46785, but that turned out to be
unsound, because the there's no guarantee that the top-level function
must be alive while import() can still be initiated from that
function, since V8 could discard the top-level function and only keep
inner functions alive, so relying on the top-level function to keep
the CompiledFnEntry alive could result in use-after-free which caused
a revert of that fix.

With this patch we use a symbol in the host defined options instead of
a number, because with the stage-3 symbol-as-weakmap-keys proposal
we could directly use that symbol to keep the referrer alive using a
WeakMap. As a bonus this also keeps the other kinds of referrers
alive as long as import() can still be initiated from that
Script/Module, so this also fixes the long-standing crash caused by
vm.Script being GC'ed too early when its importModuleDynamically
callback still needs it.

Previously we maintain a strong persistent reference to the
ModuleWrap to retrieve the ID-to-ModuleWrap mapping from
the HostImportModuleDynamicallyCallback using the number ID
stored in the host-defined options. As a result the ModuleWrap
would be kept alive until the Environment is shut down, which
would be a leak for user code. With the new symbol-based
host-defined option we can just get the ModuleWrap from the
JS-land WeakMap so there's now no need to maintain this
strong reference. This would at least fix the leak for
vm.SyntheticModule. vm.SourceTextModule is still leaking
due to the strong persistent reference to the v8::Module.

Replace the persistent handles to v8::Module and
v8::UnboundScript with an internal reference that V8's GC is
aware of to fix the leaks.