fix($parse): making unary +/- operators consistent, not allowing ++/--

Fixes #10371

* makes `+undefined` evaluate as a numeric operator the same as `-undefined`
* allows `+` to be followed by another unary operator like `-` already does
* no longer parses `--i` or `---------i` which were previously parsed as a bunch of negates

That last one is a breaking change but I assume not many people use such things. If anyone it is probably people using `--i` and then spending far too much time trying to figure out why the decrement operator isn't working right...

I'm half tempted to do the opposite change to be more restrictive (make `-` more like `+`) but that would be a bigger breaking change.