`gh stack` creates queues, not stacks

[gholms]

Stacks are "first-in, last-out," while PRs which use the stack feature merge using a "first-in, first-out" queue process. Perhaps it would be worth choosing a different name to avoid confusion while we're still pre-GA.

[gholms]

I couldn't tell if this should be a bug or a discussion, so I took my best guess. Feel free to move it if necessary!