[Experimental, WIP] In-game scene editor

[4ian]

• Add an "EmbeddedGameFrame" that displays a preview. This preview is in "game edition" mode, which for now does nothing apart from pausing the game and rendering the initial state of objects.
  • This is a working preview so it can actually be played using the debugger.
  • This works as a single iframe that is never moved at the background of the editor. Not moving it is important so that it stays loaded.
• When the scene tab is changed in the editor, the editor is asking the "preview" to change scene (all previews, but only the "in game edition" previews follow the request).
  • Other previews can be run as usual.

TODO:

• Reset/reload the embedded game frame if the game hot reloading failed.
• Create a context for switching the scene and attaching the game frame?
• Add an option to go back to the old editor (make this "editor" an opt-in choice)
• Probably anything related to actual edition in another PR.

Some things could already be backported in smaller PRs:

• Reworked debugger client/server to add a "status" to know if the game is paused or not. This fixes a bug where the Debugger tab would show something as run when it is paused.
• Refactored prepareExporter method outside of a class.
• async/await-ified hot reloading function.
• async/await-ified launchPreview function in editor.
• allow RuntimeScene render() method to run objects/layer pre-render methods.