digitallinguistics/spec: v0.27.0

This ia major release which restructures the Lexeme schema to align more closely with the linguistic definition of a lexeme (as opposed to a lexical entry). The Lexeme now has a forms field, and many of the properties of the Lexeme object have now been moved to the Lexeme Form object. Two new schemas have been added to accommodate these changes: Sense and LexemeForm. While both schemas will always appear as subitems of the Lexeme, they were given their own separate schema documents for greater readability and maintainability.