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.

tests/Encoder/TimestampLastCombCodecTest.php

@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 namespace Ramsey\Uuid\Test\Encoder;
 
 use PHPUnit\Framework\MockObject\MockObject;
@@ -62,7 +65,7 @@ class TimestampLastCombCodecTest extends TestCase
                     'time_hi_and_version' => '11e1',
                     'clock_seq_hi_and_reserved' => '9b',
                     'clock_seq_low' => '21',
-                    'node' => 'ff6f8cb0c57d'
+                    'node' => 'ff6f8cb0c57d',
                 ]
             );
         $this->codec->decode('0800200c-9a66-11e1-9b21-ff6f8cb0c57d');
@@ -80,7 +83,7 @@ class TimestampLastCombCodecTest extends TestCase
                     'time_hi_and_version' => '11e1',
                     'clock_seq_hi_and_reserved' => '9b',
                     'clock_seq_low' => '21',
-                    'node' => 'ff6f8cb0c57d'
+                    'node' => 'ff6f8cb0c57d',
                 ]
             );
         $this->codec->decodeBytes((string) hex2bin('0800200c9a6611e19b21ff6f8cb0c57d'));