Allow providing manual signature hint to commands

tests/CommandTest.php

@@ -31,8 +31,8 @@ class CommandTest extends \PHPUnit\Framework\TestCase
         $command = new Command("test", function (string $a, ?int $b, bool $c = false) { });
         $expect = [
             'a' => 'string',
-            'b' => 'int',
-            'c' => '?bool'
+            'b' => '?int',
+            'c' => 'bool=false'
         ];
         $this->assertEquals($expect, $command->parameters());
     }