Special Town Meeting has been set for April 6  for voters to decide whether to rezone land at 129 Parker St.  to accommodate a proposed commercial development. Southborough-based Capital Group Properties  has already begun demolition of some buildings at the site, the former home of Digital Equipment Corp.,  and has proposed building a mixed-use development of about 700,000  square feet. The meeting will start at 10 a.m.  in the Fowler School.