Add node flags for blank lines before and after a node

When inserting a node (e.g. during a code fix or refactoring), it would be very helpful if there were a way to say "ensure that there is a blank line following this node", rather than inserting one, since inserting one might result in a pair of blank lines. The determination of whether or not to actually insert a linebreak would be made in the printer.

首先阅读 issue 中描述的 printer 行为，并跟踪代码修复或重构期间节点是如何插入的。当节点级 flag 可以请求在节点之前或之后添加空行，并且 printer 避免添加重复的空行时，这项工作就完成了。