Add "only mergeable as a full stack" option?

[Sevans711]

I tried using stacked PRs to split a change into a few logically distinct pieces to make them easier to review. But, merging them one at a time would leave main in an undesirable state. It would be really helpful to have a way to mark "only mergeable as a full stack".

Example:

• PR1 (branched off of main): make more dependencies optional in pyproject.toml
• PR2 (branched off of PR1): add CI coverage to ensure missing optional dependencies only lead to failures in the optional features that would actually need them.
• PR3 (branched off of PR2): improve error messages in cases where optional dependencies are missing.

Is this a misuse of the stacked PRs feature? I originally thought something like this was a perfectly reasonable use-case. But, it doesn't seem to be fully supported, because PR1 could be merged by itself accidentally, once approved.

Combining all these changes into a single PR is also undesirable because this logical split is much easier to review. These changes touch many different files throughout the codebase; splitting them like this makes it clear which file changes are relevant to each logical step (feature / CI coverage / improved messaging).