refactor: add a TemplateFile DTO

Add a data transfer object that stores the source file name, the file
name to generate and an optional path to create for each file to
generate.

As I'm using a `readonly` class for the DTO, this required updating PHP
to 8.2.

nix develop --command php bin/build-configs run

flake.nix

@@ -11,10 +11,7 @@
       in
       {
         devShell = with pkgs; pkgs.mkShell {
-          buildInputs = [
-            just
-            php
-          ];
+          buildInputs = [ php82 ];
         };
       });
 }