parser: enforce statement end after commands

[Franklin-Qi]

Reject bare { after commands instead of treating it as nested brace depth.

Fixes #36

[Alonely0]

Notice these changes make it so now the proper grammar fails: { print 1; { print 2 } }. Please, also add a test case for it so we don't regress in the future.

[Franklin-Qi]

请注意，这些更改导致正确的语法现在会出错：{ print 1; { print 2 } }。请同时添加一个测试用例，以免将来出现倒退。

Additional tests have been added: { print 1; { print 2 } }