[ts][pixi-v7][pixi-v8] Add option to sync attachment visibility to slot object

[davidetan]

Pixi users have often requested an option to sync attachment visibility with slot objects.
See:

• https://esotericsoftware.com/forum/d/27945-pixi-sprite-as-child-of-slot-in-spine-pixi-v7/7
• [ts][pixi] Adding a child Pixi object in a Slot that follows its transform #2539 (comment)

A solution was initially proposed here, but the user eventually agreed to use the afterUpdateWorldTransforms method, as demonstrated below:

const slot = spineboy.skeleton.findSlot("empty");
spineboy.afterUpdateWorldTransforms = () => pixiObject.visible = Boolean(slot.attachment);

Proposal

We should consider modifying the addSlotObject method to include an options object. This would allow for future extensibility while addressing this specific need. The proposed signature is:

addSlotObject(slotRef: number | string | Slot, pixiObject: DisplayObject, options?: { syncToAttachment?: boolean }): void

By default, syncToAttachment would be set to false to avoid breaking changes.

Additionally, it may be necessary to expose a method to modify the options associated with a slot object after its creation.

[davidetan]

Implemented in 2c00d86 for v8 e 6f8ed12 for v7.

The signature is slightly changed to:

addSlotObject (slotRef: number | string | Slot, pixiObject: Container, options?: { followAttachmentTimeline?: boolean }): void

It will be released in 4.2.71 soon.