Add composer.json with league/commonmark, guzzlehttp/guzzle, phpmailer/phpmailer; wire autoloader into bootstrap; document de-librairisation strategy and PHP extension setup

phpstan.neon

@@ -7,5 +7,5 @@ parameters:
         - app/src
     bootstrapFiles:
         - app/bootstrap.php
-    scanFiles:
-        - app/src/Parsedown.php
+    scanDirectories:
+        - app/src