smug: add project session setting

Add an optional session setting so that session names can be different
than the smug project name. This also enables using template variables
when naming sessions.

tests/modules/programs/smug/project.yml

@@ -0,0 +1,5 @@
+root: ~/project
+session: project-${worktree}
+windows:
+- layout: tiled
+  name: src

tests/modules/programs/smug/settings.nix

@@ -53,11 +53,23 @@
           ];
         };
 
+        project = {
+          root = "~/project";
+          session = "project-\${worktree}";
+          windows = [
+            {
+              name = "src";
+              layout = "tiled";
+            }
+          ];
+        };
+
       };
     };
   };
 
   nmt.script = ''
     assertFileContent home-files/.config/smug/blogdemo.yml ${./blogdemo.yml}
+    assertFileContent home-files/.config/smug/project.yml ${./project.yml}
   '';
 }