Support flattening top level sequences

[tjlytle]

Description

Failing test as requested from this post.

#[Field(flatten: true)]
#[SequenceField(arrayType: Coordinate::class)]
public array $coordinates;

Serializes as expected into an array of objects, but does not use the defined SequenceField when deserializing.

[Crell]

That was exciting to figure out. I didn't even realize that flattening a single top level array would do that (turn into an array rather than an object). But once I found the issue, the fix was nice and small. And blows open some new functionality people have been asking for. 😄

Also, it's a place where we want PHP 8.4... In time...