v1.2.2: first class PixiJS v8 support

[reececomo]

Improves PixiV8 support by updating library type-hinting (and naming conventions) to Container from DisplayObject.

What's changed?

• improves PixiV8 support by updating types to reflect common between v6, v7 and v8
• Replace references to PIXI.DisplayObject in types with PIXI.Container
• renamed Action.runOnChildWithName(action, name) to Action.runOnChild(nameOrLabel, action)
  • flipped argument order to be more expressive (sorry SKAction.runOnChild(_: withName:) 💔)
  • added under-the-hood support for v8's getChildByLabel() when available to minimize deprecation warnings and rename
  • changed error reporting behavior to report a TypeError when target can'wt have children, or ReferenceError when no child was found
  • no functional changes
• Added PIXI 6.1, 7.2 and 8.1 to test matrix to validate changes