Add extra syntax error cases

Author: jamesls

tests/syntax.json

@@ -95,6 +95,14 @@
       {
         "expression": "!",
         "error": "syntax"
+      },
+      {
+        "expression": "@=",
+        "error": "syntax"
+      },
+      {
+        "expression": "@``",
+        "error": "syntax"
       }
     ]
   },