Support OSC 4 ; color ; color spec

[Tyriar]

Original request: microsoft/vscode#104291

Ps = 4 ; c ; spec ⇒  Change Color Number c to the color
          specified by spec.  This can be a name or RGB specification as
          per XParseColor.  Any number of c/spec pairs may be given.
          The color numbers correspond to the ANSI colors 0-7, their
          bright versions 8-15, and if supported, the remainder of the
          88-color or 256-color table.

          If a "?" is given rather than a name or RGB specification,
          xterm replies with a control sequence of the same form which
          can be used to set the corresponding color.  Because more than
          one pair of color number and specification can be given in one
          control sequence, xterm can make more than one reply.