Shorter enable environment variable

The current env variable to enable the layer is ENABLE_VK_LAYER_TORKEL104_libstrangle=1 which is very obvious and clear but also long and verbose.

Mangohud uses simple MANGOHUD=1 and MANGOHUD_DISABLE=1 variables. Could libstrangle adopt a similar pattern (e.g. STRANGLE=1)?

I'm not sure whether implicit layers allow using multiple variables but that would probably be desirable for backcompat.