Move '\' character to class b

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/frontend/scanner.cpp

@@ -46,7 +46,7 @@ public:
         set('_', 'a');
 
         // characters that can be used to create ids of group b
-        for (unsigned char b : {'=', '<', '>', '@', '^', '|', '&', '~', '+', '-', '*', '/', '$', '%', '?', ';'})
+        for (unsigned char b : {'=', '<', '>', '@', '^', '|', '&', '~', '+', '-', '*', '/', '\\', '$', '%', '?', ';'})
             set(b, 'b');
 
         // punctuation