Implement PRGSLOT

SMILEBASIC/compiler.d

@@ -226,6 +226,7 @@ class Compiler
         addSystemVariable("RESULT", new Result());
         addSystemVariable("HARDWARE", new Hardware());
         addSystemVariable("MILLISEC", new MilliSecond());
+        addSystemVariable("PRGSLOT", new ProgramSlot());
     }
     void addSystemVariable(wstring name, SystemVariable var)
     {

SMILEBASIC/program.d

@@ -518,7 +518,7 @@ class Program
 {
     Slot[] slot;
     bool PRGEDITused;
-    int currentSlot;
+    int currentSlot = -1/*PRGSLOT default value*/;
     int slotSize = 4;
     this()
     {

SMILEBASIC/systemvariable.d

@@ -131,3 +131,11 @@ class MilliSecond : SystemVariable
             return Value(cast(int)SDL_GetTicks());
         }
 }
+class ProgramSlot : SystemVariable
+{
+    @property
+        override Value value()
+        {
+            return Value(vm.petitcomputer.program.currentSlot);
+        }
+}