Issue 2854: parser.yy: remove STRING_IDENTIFIER token

Issue 2854: parser.yy: remove STRING_IDENTIFIER token

A STRING_IDENTIFIER always was a special case of a MARKUP_IDENTIFIER
without being treated equally at top level.  That led to craziness
like

xxx=\markup "test"
\xxx

being invalid, while

xxx=\markup \italic "test"
\xxx

was valid code.  This change just removes the STRING_IDENTIFIER
category altogether.  Where it was used previously, MARKUP_IDENTIFIER
is accepted now syntactically, and where the semantics indeed require
a plain string, an error is generated explicitly in the action instead
of leaving it to the parser to complain about bad grammar.