FR: create bookmark pointing to new change

[sersorrel]

Is your feature request related to a problem? Please describe.
In a megamerge workflow, I usually find myself in situations like this (diagram copied shamelessly from https://v5.chriskrycho.com/journal/jujutsu-megamerges-and-jj-absorb/):

            m --- n
           /       \
a -- [main] -- c -- [merge] -- @

If I want to start a new independent change, I can run something like jj new -A main -B merge --no-edit, to get something like:

            m --- n
           /       \
a -- [main] -- c -- [merge] -- @
           \       /
            -- w --

However, I like to leave a wip bookmark on the change I'm working on (so I can remember what I was doing after lunch, so that my shell prompt that shows me wip's description works properly, ...), and currently I have to manually jj b s wip -r w to do that. (actually just jj wip w, but I still have to read through the output of jj new to find the newly-created change ID, which is not always obvious.)

Describe the solution you'd like
I'd like to be able to run something like jj new -b wip to create/move the bookmark wip to the newly-created change in a single command.

Describe alternatives you've considered
I wanted to create an alias for this (jj newip or something), but I don't think it's easily possible, because you can't pass a template to jj new – I'd have to do some error-prone parsing (or inspect the op log, I guess?) to find the new change ID. Being able to jj new -T 'new_change_id()' or something like that would also probably solve this.

Additional context

If you're curious what I have in my prompt:

jj log --color always --no-graph -r '@|bookmarks(exact:"wip")' -T 'concat(separate(" ", label("bookmark", if(current_working_copy, "@")), label("bookmark", if(bookmarks, bookmarks.filter(|b| b.name() != "wip"))), format_short_change_id_with_hidden_and_divergent_info(self), if(empty, label("empty", "(empty)")), if(description, label("description", description.first_line()), label(if(empty, "empty"), description_placeholder))), "\n")'

It comes out looking like:

username@host:/wherever
@ szrkxrrp (empty) (no description set)
thunar-trash xnltntxt common: Patch Thunar to replace "Wastebasket" with "Trash"
$

[yuja]

Since jj doesn't have a "current bookmark", I don't think jj new -b/--bookmark would make much sense. Maybe we'll need a better scripting support?

Some related discussions:

api/scripting:
#3219
#3262
#3814

topic/rebase:
#3402
#4708