ref(home/shell): make it easier to share common things between shells

I'm considering trying again fish, and there are a number of things that
should be common between zsh and fish (aliases, environment variables,
...).

Instead of duplicating these settings multiple time, I'm consolidating
the shell configurations under `home/shell`, and I can set the shell I
want to use with `my.home.shell.name`.

The first step is to move the modules for fish and zsh under
`home/shell`, add an interface to pick which one I want to use, and
modify the `host/home.nix` configuration to keep using zsh with the new
interface.

Change-Id: Idb66b1a6fcc11a6eeaf5fd2d32dd3698d2d85bdf
Reviewed-on: https://cl.fcuny.net/c/world/+/455
Tested-by: CI
Reviewed-by: Franck Cuny <franck@fcuny.net>

1 files changed, 0 insertions, 0 deletions

diff --git a/home/zsh/gerrit.zsh b/home/shell/zsh/gerrit.zsh
index 9766b2f..9766b2f 100644
--- a/home/zsh/gerrit.zsh
+++ b/home/shell/zsh/gerrit.zsh