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

src/DataTransferObject/TemplateFile.php

@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace OliverDaviesLtd\BuildConfigs\DataTransferObject;
+
+readonly final class TemplateFile
+{
+    public function __construct(
+        public string $data,
+        public string $name,
+        public string|null $path = null,
+    ) {
+    }
+}