Tweak backslash escaping

The initial implementation was testing the byte value rather than
the converted value, so backslashes were getting through in high
ASCII strings.  PETSCII and C64 screen codes don't really have a
backslash so it's not really an issue there.

The new implementation handles high ASCII correctly.  The various
201n0-char-encoding-x regression tests have been updated to verify
this.

SourceGen/SGTestData/Source/20120-char-encoding.S

@@ -144,5 +144,20 @@ allbytes
         !hex    e0e1e2e3e4e5e6e7e8e9eaebecedeeef
         !hex    f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff
 
-end
+end     nop
+        jmp     skip_bs
+
+        !text   "ASCII escapes \: \\ \r \n \",$22," \",$27," \t"
+        !byte   $80
+        !pet    "PETSCII escapes \: \\ \r \n \",$22," \",$27," \t"
+        !byte   $80
+        !scr    "SCR escapes \: \\ \r \n \",$22," \",$27," \t"
+        !byte   $80
+        !xor $80 {
+; Can't include string delimiter due to code gen limitation.  Do "\0" instead.
+            !text   "HIGH escapes \: \\ \r \n \",$30," \",$27," \t"
+        }
+        !byte   $80
+
+skip_bs nop
         rts