feat(php): 添加对联合类型的支持

- 在编译器基类中处理UnionType情况，将其转换为mixed类型
- 添加nullable类型属性的测试用例
- 实现对union类型参数的正确解析和处理逻辑

src/Php/CompilerBase.php

@@ -1004,6 +1004,9 @@ class CompilerBase extends \PhpAot\Core\Translator
                 if ($param->type instanceof NullableType) {
                     $type = $param->type->type;
                     $nullable = true;
+                } elseif ($param->type instanceof UnionType) {
+                    $type = 'mixed';
+                    $nullable = false;
                 } else {
                     $type = $param->type === null ? '' : $param->type;
                     $nullable = false;

tests/aot/prop-004.phpt

@@ -0,0 +1,18 @@
+--TEST--
+Property with nullable type
+--FILE--
+<?php
+class Foo
+{
+    public function __construct(public string|Stringable $value)
+    {
+    }
+}
+
+function main() {
+    $foo = new Foo('Continue');
+    var_dump($foo->value);
+}
+?>
+--EXPECT--
+string(8) "Continue"