Add clearer error messages when using set/get incorrectly.

It's quite easy for users to forget to use the colon before their
setters and getters, but in this situation error messages are unclear about
the real nature of the problem.

This commit adds a check for malformed set/get statements and provides
error messages containing more direct guidance to users.

Closes #91216

modules/gdscript/gdscript_parser.cpp

@@ -1095,8 +1095,36 @@ GDScriptParser::VariableNode *GDScriptParser::parse_variable(bool p_is_static, b
 	}
 
 	complete_extents(variable);
+
+	bool malformed_setget_found = false;
+
+	if (p_allow_property && match(GDScriptTokenizer::Token::IDENTIFIER)) {
+		StringName identifier_name = previous.get_identifier();
+		if (identifier_name == SNAME("set") || identifier_name == SNAME("get")) {
+			push_error(vformat(R"(Expected ":" before "%s".)", identifier_name));
+			malformed_setget_found = true;
+		} else {
+			push_error(R"(Expected end of statement after variable declaration, found "Identifier" instead.)");
+		}
+	}
+
 	end_statement("variable declaration");
 
+	if (p_allow_property && !malformed_setget_found && match(GDScriptTokenizer::Token::INDENT)) {
+		if (match(GDScriptTokenizer::Token::IDENTIFIER)) {
+			StringName identifier_name = previous.get_identifier();
+			if (identifier_name == SNAME("set") || identifier_name == SNAME("get")) {
+				malformed_setget_found = true;
+			}
+		}
+
+		if (malformed_setget_found) {
+			push_error(vformat(R"(Expected ":" at end of variable declaration when using "set" or "get")"), variable);
+		} else {
+			push_error(R"(Unexpected "Indent" after variable declaration.)");
+		}
+	}
+
 	return variable;
 }