Fix GH-8232 - always reference classes in `var_export()` via their FQCN

Closes GH-8233

This fix corrects a behavior of `var_export()` that was mostly "hidden" until PHP 8.1 introduc

Fix GH-8232 - always reference classes in `var_export()` via their FQCN

Closes GH-8233

This fix corrects a behavior of `var_export()` that was mostly "hidden" until PHP 8.1 introduced:

* properties with object initializers
* constants containing object references
* default values of class properties containing `enum`s

Since `var_export(..., true)` is mostly used in conjunction with code generation,
and we cannot make assumptions about the generated code being placed in the root
namespace, we must always provide the FQCN of a class in exported code.

For example:

```php
<?php

namespace MyNamespace { class Foo {} }

namespace { echo "<?php\n\nnamespace Example;\n\n" . var_export(new \MyNamespace\Foo(), true) . ';'; }
```

produces:

```php
<?php

namespace Example;

MyNamespace\Foo::__set_state(array(
));
```

This code snippet is invalid, because `Example\MyNamespace\Foo::__set_state()` (which
does not exist) is called.

With this patch applied, the code looks like following (valid):

```php
<?php

namespace Example;

\MyNamespace\Foo::__set_state(array(
));
```

Ref: https://github.com/php/php-src/issues/8232
Ref: https://github.com/Ocramius/ProxyManager/issues/754
Ref: https://externals.io/message/117466

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