Unify the lexer's idea of words and commands across all modes.

Unify the lexer's idea of words and commands across all modes.

A "word" (a string recognized even when not quoted) and a "command"
(something starting with \ and followed by letters and other folderol,
indicating a Scheme control sequence or similar) get the same syntax
in all modes:

A "word" is a sequence of alphabetic characters possibly containing
single dashes or underlines inside (not at the beginning or end).

A "command" is a "word" preceded by a backslash.