add himalaya-watch@.service

As discussed in
<https://github.com/nix-community/home-manager/issues/5069>.

I set `ExecStart=%install_dir%/himalaya` so when packaging himalaya
people nee to explicitly set the path to himalaya (i.e. `sed
's:%install_dir%:/usr/bin:' assets/himalaya-watch@.service`). This is
done automatically in `install.sh` if `$PREFIX` is `/usr`, Otherwise
the packager should handle it themselves

For `nix` it would be (`sed 's:%install_dir%:$out/bin:'
assets/himalaya-watch@.service`). I don't know where it should be placed
(probably `$out/share/systemd/user` as nix will add that to
`$XDG_DATA_DIRS` which is searched by `systemctl --user`.

I swear I checked the address like 4 times before sending the email, I
have no idea how I managed to mess it up T-T. I was wondering why the
formatting was so messed up in sr.ht.

assets/himalaya-watch@.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=Email client Himalaya CLI envelopes watcher service
+After=network.target
+
+[Service]
+Type=exec
+ExecStart=%install_dir%/himalaya envelopes watch --account %i
+ExecSearchPath=/bin
+Restart=always
+RestartSec=10
+
+[Install]
+WantedBy=default.target