Add "uninitialized data" format type

This allows regions that hold variable storage to be marked as data
that is initialized by the program before it is used.  Previously
the choices were to treat it as bulk data (initialized) or junk
(totally unused), neither of which are correct.

This is functionally equivalent to "junk" as far as source code
generation is concerned (though it doesn't have to be).

For the code/data/junk counter, uninitialized data is counted as
junk, because it technically does not need to be part of the binary.

SourceGen/SGTestData/Source/20000-numeric-types.S

@@ -71,3 +71,16 @@ dref    ds      16,$85              ;has a data reference
         ds      16,$8a              ;EDIT: add visualization
 cref    ds      16,$8b              ;has a code reference
         ds      16,$8c              ;EDIT: format byte as binary
+
+; Some uninitialized data for the "uninit" op.
+        dfb     $90
+        dw      $9192
+        adr     $939495
+        adrl    $96979899
+        ds      1
+        ds      2
+        ds      3
+        ds      4
+        dfb     $80
+        ds      5
+        dfb     $80