feat: add default value support for @template annotations

[romm]

Every generic of a class had to be given during mapping, even when a sensible fallback existed for some of them. Following PHPStan and Psalm, a template can now declare a default type, making the matching generic optional:

/**
 * @template TValue
 * @template TMeta of array<string, mixed> = array<string, string>
 */
final readonly class SomePage
{
    public function __construct(
        /** @var list<TValue> */
        public array $items,

        /** @var TMeta */
        public array $meta,
    ) {}
}

final readonly class SomeClass
{
    public function __construct(
        // `TMeta` is not filled in, its default type is used
        /** @var SomePage<SomeUser> */
        public SomePage $pageWithDefaultMeta,

        // `TMeta` is filled in, overriding its default type
        /** @var SomePage<SomeUser, array{cursor: int}> */
        public SomePage $pageWithCursorMeta,
    ) {}
}

Templates declaring a default are not counted as required anymore, and the default is used as the fallback for an omitted generic, instead of the template itself.

Fixes #600