Support grammatical number in concept model

I have no idea where this fits in to the glossarist universe, but grammatical number needs to be captured just like grammatical gender: we need to be able to indicate that a designation is singular or plural.

I am adding Number after Gender in the concept model. The conventional abbreviations are `sg` and `pl`.

See https://github.com/metanorma/metanorma-iso/issues/554