Bump php to ^8.0, add tooling, linting, fixes

rector.php

@@ -0,0 +1,16 @@
+<?php
+
+declare(strict_types=1);
+
+use Rector\Config\RectorConfig;
+
+return RectorConfig::configure()
+    ->withPaths([
+        __DIR__.'/config',
+        __DIR__.'/src',
+        __DIR__.'/tests',
+    ])
+    ->withPhpSets()
+    ->withPreparedSets(deadCode: true, codeQuality: true)
+    ->withImportNames(removeUnusedImports: true)
+    ->withTypeCoverageLevel(0);