Add support or alternative for steinmetz

[eclecticfluff]

I wish to be able to typeset polar complex numbers in the style given by the steinmetz package, but unfortunately complex-symbol-angle = \phase does not work. I would appreciate it if the internal code could be changed for this to work, or alternatively another key be added explicitly for wrapping the phase component of polar complex numbers in an arbitrary macro. Ideally, a new function could be created within siunitx to match this style of typesetting, adding options to better match font and scale in a smarter way than blindly enclosing the bounding box. This would likely take considerably more effort to implement, however, so I understand if contributor motivation isn't there for it.

[josephwright]

I will see what Enrico thinks here

[eg9]

The steinmetz package was written in order to implement a representation of complex numbers that's common in engineering fields, so I guess it would be good to allow using it with siunitx.

I'm not sure, however, about how to integrate it with siunitx, because the single command \phase actually only typesets the measure of the angle.