prepared QTSW flag for proper screen code -> PETSCII conversion

c64/kernal/screen/screen_check_toggle_quote.s

@@ -0,0 +1,21 @@
+// #LAYOUT# STD *        #TAKE
+// #LAYOUT# X16 *        #IGNORE
+// #LAYOUT# *   KERNAL_0 #TAKE
+// #LAYOUT# *   *        #IGNORE
+
+//
+// Helper routine - check character in .A, toggles quote flag if needed
+//
+
+screen_check_toggle_quote:
+
+	cmp #$22
+	bne !+
+
+	lda QTSW
+	eor #$80
+	sta QTSW
+
+	lda #$22                           // restore previous .A valuee
+!:
+	rts