Update coding style to include PSR-12, among other options

This also includes heavy use of slevomat/coding-standard to apply
various checks to the code, based on maintainer (me) preference.

src/Provider/Node/FallbackNodeProvider.php

@@ -38,7 +38,7 @@ class FallbackNodeProvider implements NodeProviderInterface
     /**
      * @inheritDoc
      */
-    public function getNode(): ?string
+    public function getNode()
     {
         foreach ($this->nodeProviders as $provider) {
             if ($node = $provider->getNode()) {