Add bindings for `AllowJavascriptExecutionScope`

[andreubotella]

In denoland/deno#12505 it came up that trying to run Javascript code from v8::ValueDeserializerImpl::read_host_object resulted in a crash because apparently that method is invoked from V8 inside a DisallowJavascriptExecutionScope. Implementing structured clone for web APIs in Deno essentially requires running JS code in the implementation of that method, and so bindings to v8::Isolate::AllowJavascriptExecutionScope would be needed to make that possible.

[piscisaureus]

@piscisaureus cc self