src: only expose per-isolate symbols wherever necessary
Currently the all-in-one internalBinding('symbols')
is only used in the MessagePort prototype setup.
Until we actually need to share these symbols across multiple files,
only expose them wherever necessary and make them as local as
possible.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines