feat: parser now writes, added missing fields

updated docs, example, extensive use of Traits with better handling.
valid level 9 phpstan codebase.

src/Traits/HasLanguage.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace Ivuorinen\BusinessDataFetcher\Traits;
+
+trait HasLanguage
+{
+    /**
+     * @see getLanguageString()
+     * @var string|null $language Two letter language code
+     *                            (e.g. 'fi', 'sv', 'en')
+     */
+    public ?string $language;
+
+    /**
+     * Get the language code as a string.
+     */
+    public function getLanguageString(): string
+    {
+        return match ($this->language) {
+            'fi' => 'finnish',
+            'en' => 'english',
+            'sv' => 'swedish',
+            default => 'unknown:' . $this->language,
+        };
+    }
+}