Add toon shader from bevy_toon_shader

[alice-i-cecile]

Fixes #641.

To do:

• do asset preprocessing to swap out all materials from loaded scenes to use the equivalent toon-shaded material. Use bevy_scene_hook for this IMO. Use https://github.com/DGriffin91/bevy_mod_standard_material

Blockers:

• it does not appear possible to adjust the overall light intensity (No way to set the light intensity of the toon shader tbillington/bevy_toon_shader#2)