Allow trailing comma in map literal

src/vm/wren_compiler.c

@@ -1853,6 +1853,9 @@ static void map(Compiler* compiler, bool allowAssignment)
     {
       ignoreNewlines(compiler);
 
+      // Map with trailing comma.
+      if (peek(compiler) == TOKEN_RIGHT_BRACE) break;
+
       // Push a copy of the map since the subscript call will consume it.
       emit(compiler, CODE_DUP);
 

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