feat: model viewer / converter GUI

[craftablescience]

• [x] Model Viewer
  • [x] View models
  • [x] Editor camera
  • [x] Convert models between formats
• [x] Make stb_image an interface library
• [x] Delete unused Discord library code
• [x] Fork ImGui and add cmake support
• [x] Fix several bugs
• [x] Create custom model format
• [x] Add minimally necessary wrapper functions to deal with tinyfiledialogs functions sanely
• [x] Add assertions that display a popup dialog in debug
• [x] Add a material factory so meshes can refer to materials within their JSON
• [x] Capitalize all classes
• [x] Use assert_cast instead of dynamic_cast