Add support for reviewing Stack PRs

[zhengbuqian]

https://docs.github.com/en/pull-requests/get-started/about-stacked-prs

would be great if when reviewing a stack of PRs, we can quickly navigate between PRs of a same stack. maybe in "CHANGES IN PULL REQUEST #xxx" we can have "CHANGES IN STACK #xxx" and have the Files/Commits of each individual PRs ready to be reviewed/commented.

such as:

Changes in Stack #xxx
  [author avatar] PR 1 title
    > Files # expand to see list of changed files, comments sent to PR 1
    > commits
  [author avatar] PR 2 title
    > Files # expand to see list of changed files, comments sent to PR 2
    > commits
  ...