Add signed-decimal operand formatting

This allows signed decimal operands to be formatted as such, e.g.
"LDA #$FE" becomes "LDA #-2".  This can be applied to immediate
operands and to numeric data pseudo-ops.

Not all assemblers support this in all situations.  The asm generators
will output unsigned decimal operands if so.

The 20020- and 20022-operand-formats tests have been updated.

SourceGen/SGTestData/Source/20020-operand-formats.S

@@ -62,7 +62,7 @@
         lda     #$fe
         lda     #$ff
 
-        jmp     end
+        jmp     next1
 
 ; Continuing with ASCII
 :ascii
@@ -96,4 +96,34 @@
         dfb     $fd
         dfb     ","
 
-end     rts
+; Signed-decimal tests.  All operands should be formatted as signed.  This
+; will require hand-editing or tweaking the operand editor.
+next1   nop
+        lda     #$00                    ;these should format as signed
+        lda     #$01
+        lda     #$7f
+        lda     #$80
+        lda     #$ff
+
+        lda     $01                     ;these should appear unsigned
+        lda     $fe
+        lda:    $0001
+        lda     $fffe
+        jmp     :skipdat1
+
+        dfb     $00                     ;all of these should appear signed
+        dfb     $01
+        dfb     $7f
+        dfb     $80
+        dfb     $ff
+        dw      $0001
+        dw      $fffe
+        dw      $0200                   ;big-endian
+        dw      $fdff                   ;big-endian
+        adr     $010203
+        adr     $fffefd
+        adrl    $01020304
+        adrl    $fffefdfc
+:skipdat1
+
+        rts

SourceGen/SGTestData/Source/20022-operand-formats.S

@@ -37,4 +37,14 @@
         ldal    $000068     ;'h'
         ldal    $0000c8     ;"h"
 
+; Signed-decimal tests.
+        nop
+        lda     #$0000
+        lda     #$0001
+        lda     #$7fff
+        lda     #$8000
+        lda     #$ffff
+
+        ldal    $fffefd
+
         rts