Disallow {0}² and {0}³ in long units

Many vetters are just checking off on the root versions of power2 and power3, namely {0}² and {0}³. However, these are inappropriate for long names, since you get names like "kilometer²" or "meter per second²". The main reason people seem to pick these are when their languages have gendered units. (But many languages with gender get around this by using abbreviations

I propose that we make these into errors for any language that has gender information, and warning for those without gender information.