move <a-assets> within <a-scene>

[ngokevin]

Set up work for preloading and later on mixin features.

[dmarcos]

With multiple scenes we might want to share assets between them. Imagine you divide multiple levels in multiple separate scenes. How are we going to handle that?

[ngokevin]

I pre-commented on that in the issue: #908 (comment)

Some reasons why I would move <a-assets> into the <a-scene> in the meantime:

• Focus on getting good single scenes first, then maybe think about multiple scenes. The multiple scene use case is hypothetical atm.
• Enables work on preloading assets and mixin stylesheets
• That's with the assumption that multiple levels would utilize multiple scenes/canvases. I don't think multiple scenes are necessary with that case.
• Global assets means asset/mixin scope pollution between scenes.
• Global assets raises questions on whether unrelated assets from one scene block another scene.
• Raises question on whether the scene blocks on the global assets or just the local one.
• More code to handle global assets for multiple scenes.
• Flexible choice as IIRC we currently don't have anyone using a single assets for multiple scenes, and if we wanted to down the road, we could always layer on extra logic for shared/global assets.