Add a /nick command to change one's local name in chat

[JasonWeill]

Problem

I want to change the name that shows up as me in Jupyter AI chat.

Proposed Solution

Add a /nick command that lets the user change their name to the value of its only parameter, including spaces.

The user's name should appear as "Specified name (me)" in the chat UI. " (me)" cannot be customized or removed.

Running /nick with no parameter will reinstate their auto-selected name.

Jupyter AI should respond to /nick with an informative message, such as, "OK, I will now call you (your new name)" or "OK, I will now call you by the default name of (default name)".

/nick is chosen for parity with IRC, which uses this command for setting one's nickname. Discord uses the same command, though with a slightly more verbose syntax.

Additional context

Follow up to #75.

See also jupyterlab/jupyterlab#14327.