Allow letter + number for StyleBase.category_number #286

Currently, StyleBase.category_number is an integer. The BJCP mead and cider categories are M1–M4 and C1–C2, so they can’t be represented. Changing the type would break existing documents, so per ADR-0003 this needs a new string property plus deprecation of the old one. Related: style_letter allows a space, and allows only one character.