add ISO warnings to model library

[fwild]

Move all warnings to the model library, as separate groups.

• [ ] “ISO 7010 - Safety”: E-signs.zip

• [ ] “ISO 7010 - Fire”: F-signs.zip

• [ ] “ISO 7010 - Mandatory”: M-signs.zip

• [ ] “ISO 7010 - Prohibition”: P-signs.zip

• [ ] “ISO 7010 - Warning”: W-signs.zip

Then remove all Assets/MirageXR/ContentTypes/Warning/SymbolSprites/*.png and delete the player-only warning augmentation code.