[naga hlsl-out] Generate SV_StartVertexLocation and SV_StartInstanceLocation when supported to handle non-zero base_instance/vertex.

[Elabajaba]

Is your feature request related to a problem? Please describe.
DX12/HLSL is dumb and always gives you 0 in the shader for base_vertex/base_instance no matter what value you set it to in the draw call. Microsoft finally added 2 new keywords in shader model 6.8 to get the offset base_vertex/base_instance values.

Describe the solution you'd like
Microsoft finally added 2 new keywords to get the start/base vertex/instance location in shader model 6.8, so we should use those and disable the shader param injection workaround in wgpu if SM 6.8+ is in use.
https://github.com/microsoft/DirectX-Specs/blob/master/d3d/HLSL_ShaderModel6_8.md#extended-command-information

Additional context
#2471