Pull request stacks [Public Preview]

### Value Prop

We're making it easier to land changes by enabling you to break up the changes into separate pull requests that are stacked on top of each other. Each pull request in the stack is aware of the other pull requests in the stack and what its ultimate target branch is (so the appropriate branch protections and rules are enforced) and individual parts (or the entire stack) can be merged as soon as they are ready.

Because each part of the stack is a just a pull request, there is a low barrier to entry, does not require major changes to your team's workflows, and works with existing apps and tools.

### Expected Outcome

1. Pull requests can be stacked on top of each other, with the appropriate branch protections and rules applied on each pull request in the stack.
2. All or part of the stack can be merged once the necessary requirements have been met.
3. Works with existing app and tools that understand branches and pull requests.