Simplified shader versions for qsb tool

Currently to bake the shaders with qsb tool you use something like:

qsb --glsl "100 es,120,150" --hlsl 50 --msl 12 -o shader.frag.qsb shader.frag

I propose that we add "-qt6", "default", "-cross-platform" or similar to create set of shader versions that we recommend, so common way would become:

qsb --qt6 -o shader.frag.qsb shader.frag

Reasoning:

• Users don't need to remember exact options to be able to create cross-platform shaders.
• As the usage of qsb files increases, it it better to have internet filled with similar cross-platform versions than ones baked with misc options.
• At some point of Qt 6.x we may decide to update what versions CMake qt6_add_shaders generates, and at the same time could update these qsb tool defaults -> Baking with new Qt versions will automatically output updated versions.