Chirp use statement

[nicopap]

Currently nothing prevents someone from loading a chirp file within another chirp files.

But to make it more ergonomic, we should add a direct way of doing it.

use scene_file

Then, scene_file can be called like a template with zero arguments: scene_file!(). In addition, it should be possible to use templates defined in other files with a use scene_file/specific_template, then calling specific_template!(…).

Would load in-line scene_file.chirp.

Ideally we'd check that:

• It works correctly at initial load
• Works correctly when hot-reloading the parent chirp file
• Works correctly when hot-reloading the referred chirp file

This is blocked on bevy 0.12 asset v2