Handle double-quoted field under QUOTED state

Replaces regular expression named  DQUOTE with a new QUOTED exclusive start condition. The rules under the QUOTED state are similar to those under TOKEN state (see pull-request #25). The purpose is to prepare the lexer for introduction of a new RECUR exclusive state meant to handle possibly embedded constructs. Back to pull-request #23, recognition of ${...}, $(...), $((...)) constructs was postponed because these constructs could not be covered solely by regular expressions and required a recursive algorithm. When one of the possibly nested constructs mentioned above is identified, the lexer is supposed to enter the RECUR state and increment a static global recursion depth variable. The details are to be discussed later on.