This change makes LibJS correctly report a syntax error when a unary
expression is followed by exponentiation, as the spec requires.
Apparently this is due to that expression being ambiguous ordering.
Strangely this check does not seem to apply in the same way for '++' and
'--' for reasons that I don't fully understand. For example
```
let x = 5;
++x ** 2
```
Since `--5` and `++5` on it's own results in a syntax error anyway, it
seems we do not need to perform this exponentiation check in those
places.
Diff Tests:
+6 ✅ -6 ❌
The JS behaviour of exponentiation on two number typed values is
not a simple matter of forwarding to ::pow(double, double). So,
this factors out the Math.pow logic to allow it to be shared with
Value::exp.
