feat: allow trailing comma in map literal by checking right brace in parser

Add a check in the map parsing function to break when encountering a closing brace after a comma, enabling trailing commas in map literals. Include a test case verifying that maps with trailing commas parse and execute correctly.

test/language/map/trailing_comma.wren

@@ -0,0 +1,7 @@
+var map = {
+  "a": 1,
+  "b": 2,
+}
+
+IO.print(map["a"]) // expect: 1
+IO.print(map["b"]) // expect: 2