Support `eval` array notation inside attributes

[thekid]

This pull request makes it possible to use array notation inside attributes as follows:

// Currently supported
#[Callback(eval: 'fn() => ...')]
class A { }

(new XPClass(A::class))->getAnnotation('callback'); // Closure{}

// Additionally supported
#[Callback(eval: ['fn() => ...'])]
class B { }

(new XPClass(B::class))->getAnnotation('callback'); // Closure{}

// Using named arguments
#[Callback(eval: ['function' => 'fn() => ...'])]
class C { }

(new XPClass(C::class))->getAnnotation('callback'); // ['function' => Closure{}]

Together with xp-framework/reflection#35, this is a prerequisite for extracting reflection code as proposed by xp-framework/rfc#338.

---

⚠️ This does not offer full support because the XP Framework core annotation API is built around a single-value paradigm instead of multiple arguments. The following raises an exception:

// Unsupported
#[Callbacks(eval: ['fn() => ...', 'fn() => ...'])]
class A { }

(new XPClass(A::class))->getAnnotation('callbacks'); // *** Parse error

...just like PHP 8 attributes with multiple arguments do.

👉 However, this PR makes it possible to use array notation for single values or multiple named arguments without risking ClassFormatExceptions, ensuring partial compatibility.

---

It also enables us to emit simpler code in the XP Compiler, where we current have a special-case handling for when exactly one unnamed argument exists. The simplification that can be applied there is along the lines of the following:

- if (1 === sizeof($annotation->arguments) && 0 === key($annotation->arguments)) {
-   // ...
- } else {
-   foreach ($annotation->arguments as $key => $argument) {
-     // ...
-   }
- }
+ foreach ($annotation->arguments as $key => $argument) {
+   // ...
+ }

[thekid]

Released in https://github.com/xp-framework/core/releases/tag/v11.6.0