Add glue methods to get JSValue of a private reference
While working on servo/servo#27026, we've noticed calling
`GetScriptPrivate` will hit a CRASH consistently in Windows. jdm noticed
that this is caused by "can't return JSValue by value on Windows"; thus,
we need to expose these 2 glue methods so that we can handle the
`GetScriptPrivate` and `GetModulePrivate` correctly.
