b4: NixOS module

Hi everyone,

I have been using b4 for the last few days as a new contributor and it's
been a great experience so far.

It's definitely a full packed tool with lots of features but it's made
sending and rerolling patch series much easier.

I just tried the `b4 review tui` feature and that was pretty awesome
too. I did find it a but hard to see the inline Agent notes however.
I had to enter the "Reply mode" to see them rather than in the "View
mode" (these are the inline notes and not the high-level commit notes).

In any case, I run NixOS and I wanted to share my current NixOS module
for b4 in case anyone else is interested in using it that way.

I've upstreamed it to home-manager here [1] and you can see it in my
current NixOS configuration here [2].

If you have more suggestions or ideas on how I could improve the module
please let me know.

Thanks!

[1] https://github.com/nix-community/home-manager/pull/9658
[2] https://github.com/fzakaria/nix-home/blob/6f0e8ece2749db124e99fd7630cb9e8fc8b3ffaf/modules/home-manager/b4.nix