Reply to comments on Description page

After someone has reviewed my PR, I can see their review comments as part of the Description page.

However, to reply to these comments, I have to go to the file that the comment is in, and reply from there.

It'd be really useful to be able to just reply from the Description page.

This is possibly a duplicate of https://github.com/Microsoft/vscode-pull-request-github/issues/71#issuecomment-400525734 - however that issue refers to viewing the list of comments, not replying to them

首先找到 Description 页面上的审阅评论渲染逻辑，以及可从被评论文件使用的现有回复流程。追踪评论的加载和回复方式，然后使 Description 页面支持回复操作，并验证回复是否正确发布。