Fix js runtime concurrency issue. (#542)

Reference functions by their global object key instead of calling
.Extract() on the goja value, which would keep a reference to runtime
used for function registration, causing concurrency issues under
concurrent loads.

Lookup the function indentifiers in the js code AST during registration.
This disallows registring inlined (anonymous) functions, or invoking
the initializer outside the scope of the InitModule function.