Fixed issue in earlier quoted comma fix

A previous fix changed "," to (44) to avoid issues with other comma
processing. Normally this would be a safe equivalent, but has the
unfortunate side-effect of changing LD A,"," to LD A,(44) which is
a different instruction to the expected LD A,44.

Changing the parentheses to spaces keeps the same approach, and the
token separation in expressions, but without the side-effect.
Added DEFM check for a single string

DEFM used anything between the first and last set of double quotes,
even if it was an attempt to use a comma-separated list of strings.
We now check for a single string, and as Comet documents, it should
not contain embedded (double) quotes.