fix: add "either" in code 1023 message

Author: Troy Tae

src/compiler/diagnosticMessages.json

@@ -71,7 +71,7 @@
         "category": "Error",
         "code": 1022
     },
-    "An index signature parameter type must be 'string' or 'number'.": {
+    "An index signature parameter type must be either 'string' or 'number'.": {
         "category": "Error",
         "code": 1023
     },