Implement BREAK to exit REPT and FOR loops

Fixes #684

test/asm/break.asm

@@ -0,0 +1,22 @@
+FOR V, 1, 100
+      PRINTLN "- {d:V}"
+      IF V == 5
+          PRINTLN "stop"
+          BREAK
+      ENDC
+      PRINTLN "cont"
+ENDR
+WARN "done {d:V}"
+rept 1
+  break
+  ; skips invalid code
+  !@#$%
+elif: macro
+  invalid
+endr
+warn "OK"
+rept 1
+  if 1
+    break
+  no endc
+endr