Add and configure gram-editor

Add nix-gram-extensions for building extensions as derivations.
Switch to own home-manager fork until wrapping of other editors is
possible upstream.

users/julius/home.nix

@@ -11,6 +11,8 @@
     ../../home/kanshi.nix
 
     ../../home/direnv
+
+    ../../home/gram.nix
   ];
 
 }