Automatic constant merging of history is unfit for some workflows -- add `history merge` like in Fish

Related problem

For example, when I'm in zellij, and I split my terminal into multiple panes -- I might be running helix over and over in one pane, and doing git add .; git commit -m... and maybe there's a third one for cargo or bacon.

Describe the solution you'd like

In Fish and other shells, I can handle this gracefully. It keeps the histories separate until I call history merge, at which point I can get something from the history of another terminal without closing and reopening my current one. That way, the user gets the best of both worlds.

That's why I'd like a history merge equivalent in Nushell.

Describe alternatives you've considered

Using Fish :P But structured data is so gooooood, though. 😭

Additional context and details

No response

Start by locating Nushell's existing history command and the code that handles automatic history merging. Compare the requested behavior with Fish's history merge, then establish how separate terminal histories should be combined and how completion would be verified with tests.