home: git: extract 'delta' configuration

I want to be able to re-use it between different source control systems
(e.g: `jj`).

As a first step, extract it to a proper module so that I can have it
live in a single space.

modules/home/default.nix

@@ -8,6 +8,7 @@
     ./bluetooth
     ./calibre
     ./comma
+    ./delta
     ./dircolors
     ./direnv
     ./discord