Add Command->parameters() method to introspect arguments

tests/CommandTest.php

@@ -26,4 +26,14 @@ class CommandTest extends \PHPUnit\Framework\TestCase
         $this->assertEquals(true, $hit);
     }
 
-}
+    public function testCommandReflection()
+    {
+        $command = new Command("test", function (string $a, ?int $b, bool $c = false) { });
+        $expect = [
+            'a' => 'string',
+            'b' => 'int',
+            'c' => '?bool'
+        ];
+        $this->assertEquals($expect, $command->parameters());
+    }
+}