Add missing diagnostic message for private method quick fix

Fixes #37782

The code fix logic for adding missing private methods already existed
in fixAddMissingMember.ts (code 90038), but the diagnostic message
was missing from diagnosticMessages.json.

Added 'Declare private method {0}' message with code 90038 to enable
the quick fix to appear in the IDE.

Author: Pepper Pancoast

src/compiler/diagnosticMessages.json

@@ -7490,6 +7490,10 @@
         "category": "Message",
         "code": 90035
     },
+    "Declare private method '{0}'": {
+        "category": "Message",
+        "code": 90038
+    },
     "Replace '{0}' with 'Promise<{1}>'": {
         "category": "Message",
         "code": 90036