Unify String and StringName

modules/gdscript/tests/scripts/analyzer/errors/dictionary_string_stringname_equivalent.gd

@@ -0,0 +1,9 @@
+# https://github.com/godotengine/godot/issues/62957
+
+func test():
+	var dict = {
+		&"key": "StringName",
+		"key": "String"
+	}
+
+	print("Invalid dictionary: %s" % dict)

modules/gdscript/tests/scripts/analyzer/errors/dictionary_string_stringname_equivalent.out

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Key "key" was already used in this dictionary (at line 5).