Is your feature request related to a problem? Please describe.
There are two related problems here that this FR is designed to solve:

1. Naming local commits - it becomes very annoying to remember change IDs and/or revsets, and so if you name commits it can be a lot easier to write jj new <alias>
2. Identifying commits for the purposes of patch-based workflows. In a patch-based workflow such as internally at google, or gerrit, it'd be useful to be able to see in my jj log entries such as:

@  vzokzlpu msta 5 days ago crrev.com/c/5439098 a2016214
│ Commit description here

Similarly, it'd be nice to be able to write crrev.com/c/5439098

Describe the solution you'd like
I'd like to add a command jj alias. This would be almost the same command as jj branch - it would have the following subcommands:

• jj alias add <revision> <alias> --force
  • --force would move the alias to here even if it was previously defined elsewhere
• jj alias remove <alias>

Aliases would be associated with a change, and thus would persist across rewrites. The main distinction between aliases and branches are that:

• They would not be represented in git
• When you drop the commit, the alias would disappear too, rather than propogating to the parent

Similar to branches, you could write jj alias add @- foo, then later run jj new foo

Describe alternatives you've considered
Branches
When I asked about this earlier, branches are the currently recommended solution for such an approach. However, the problem is that a branch is not associated with a particular change. Consider:

◉  pvlpykyo msta 5 days ago foo 13bbdfc2
│  My local commit
◉  vzokzlpu msta 5 days ago main a2016214
│  immutable head here

If I were to write jj abandon foo, then the branch foo would instead move to main. The same thing would happen if I were to submit foo, then later run jj rebase --skip-empty.

Tags:
I considered just using git tags for this, and just calling them tags, but @martinvonz pointed out that might be confusing since tags are associated with a commit, rather than a change.

Additional context
Add any other context or screenshots about the feature request here.