improve character string handling in the lexer * lex.ll: Add calls to lexer_debug for character string patterns. Attempt to be consistent with handling of backslash characters in patterns passed to lexer_debug. (<DQ_STRING_START>\\{NL}): Handle EOF and EOB conditions explicitly. (octave_base_lexer::display_start_state): Handle DQ_STRING_START and SQ_STRING_START states.

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