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()) {

src/Provider/Node/RandomNodeProvider.php

@@ -27,7 +27,7 @@ class RandomNodeProvider implements NodeProviderInterface
     /**
      * @inheritDoc
      */
-    public function getNode(): string
+    public function getNode()
     {
         try {
             $nodeBytes = random_bytes(6);

src/Uuid.php

@@ -569,9 +569,6 @@ class Uuid implements UuidInterface
         return (int) hexdec($this->getTimestampHex());
     }
 
-    /**
-     * @inheritdoc
-     */
     public function getTimestampHex(): string
     {
         if ($this->getVersion() !== 1) {