Implement MILLISEC system variable

SMILEBASIC/compiler.d

@@ -228,6 +228,7 @@ class Compiler
         addSystemVariable("TABSTEP", new TabStep());
         addSystemVariable("RESULT", new Result());
         addSystemVariable("HARDWARE", new Hardware());
+        addSystemVariable("MILLISEC", new MilliSecond());
     }
     void addSystemVariable(wstring name, SystemVariable var)
     {

SMILEBASIC/systemvariable.d

@@ -124,3 +124,12 @@ class Hardware : SystemVariable
             return Value(cast(int)vm.petitcomputer.hardware);
         }
 }
+class MilliSecond : SystemVariable
+{
+    @property
+        override Value value()
+        {
+            import derelict.sdl2.sdl;
+            return Value(cast(int)SDL_GetTicks());
+        }
+}