PinePhone soft case pack and emblem framework

Various soft (TPU) cases for the PinePhone. `pinephone-case.stl` is the basic (single-extruder) model. It was made in Blender and derived from the official back case STEP file.

Also included are various models with emblems, intended for dual-extruder or filament-change setups. The emblems are 0.4 mm thick, and have a 0.2 mm lip that is buried in the case, in order to improve adhesion. This means you can print the emblem with PLA, while using TPU for the rest of the case. Since the emblem is only 3 layers thick at 0.2 mm, you can also just use three or four manual filament changes on a single-extruder setup.

Emblems can be semi-automatically added to the case using `case.scad`. This framework requires an emblem SCAD file that defines the `emblem_svg()` module, which generates a 2D path. `case.scad` then adds the third dimension and adhesion lip, and uses boolean operations to incorporate it into the main case. Needless to say, it is trivial to use this framework for other phones,