Operator typeof evaluates operand twice

[n1xx1]

As discussed in #945, the current implementation of typeof expr needs to first evaluate the operand to check if it's an unresolved reference (so that it can return "undefined") and then call GetValue. This is not correct.

Correct behaviour in node:

> const fn = () => { console.log('test'); }
undefined
> typeof fn();
test
'undefined'
> 

With jint:

jint> const fn = () => { print('test'); }
undefined
jint> typeof fn();
test
test
"undefined"
jint> 

The simple fix I attempted in #945 was calling Evaluate and then using _engine.GetValue to convert to a value, but it's not correct because different expressions have different GetValue implementation and it generated tons of problems.

Unfortunately I'm not proficient enough with jint internals to handle this issue, so this is only a bug report and not a PR.