Allow trailing comma in list literal

src/vm/wren_compiler.c

@@ -1816,6 +1816,9 @@ static void list(Compiler* compiler, bool allowAssignment)
     {
       ignoreNewlines(compiler);
 
+      // List with trailing comma.
+      if (peek(compiler) == TOKEN_RIGHT_BRACKET) break;
+
       // Push a copy of the list since the add() call will consume it.
       emit(compiler, CODE_DUP);
 

test/language/list/trailing_comma.wren

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