How to add a new psql command ?

Hello,

I was wondering : is it possible to add directly a new psql command which
can be run anytime in our PostgreSQL interpreter like any other normal psql
commands ?

I explain myself : in the PostgreSQL interpreter, when the command "\h" or
"\help" is written in the shell, we can see all the commands we have at our
disposal. We can also of course write our proper functions, in python
scripts for example, and run them direclty in one of our databases. But I
was wondering if it was possible to add directly a command, like \copy,
\select, .... For example, let's say I create a new command called \resume
which will call my own-made python script, and I would want this command to
be part of the list of commands we see when we type \h.

Is it even possible ? And if it's possible, how can I achieve that ?

Thanks for your attention and I thank you in advance for all the
informations you will possibly give me.

Cordially.