debugger-manager: Properly handle 0b11 tag values

Tag value 0b11 is currently used to represent <unicode-character> instances
(cf. sources/dylan/boot.dylan). This tag value needs to be recognized
by the debugger.

* sources/runtime-manager/debugger-manager/dfmc-accessors.dylan
  ($dylan-tag-unichar): New constant.
  (dylan-object?): Handle the $dylan-tag-unichar case.

* sources/runtime-manager/debugger-manager/queries.dylan
  (dylan-object-size): Handle the $dylan-tag-unichar case.

* sources/runtime-manager/debugger-manager/print-inspect.dylan
  (classify-dylan-object, dylan-object-class, dylan-object-immediate-value):
   Handle the $dylan-tag-unichar case.