That makes lyrics mode rather similar to markup mode regarding how
words are formed. {} are never considered part of words unless
enclosed in quotes. Unquoted words do not contain whitespace, braces,
quotes, backslashes, numbers or Scheme expressions. In addition, they
cannot start with * . = and | since that would mess with duration,
assignment and barcheck syntax. This removes some remaining
TeX-oriented cruft in the lexer. The set of word-non-starters might
need revisiting, but at least the regtests seem to pass.