Proposal to support passing target version when cross compiling

[etinquis]

• adds TargetVersion to CrossCompileInfo so that the caller can control the target compilation version (if 0 is passed, the behavior is as before)
• adds targetVersion param to SpirvCompilation.CompileCompute/CompileVertexFragment
• adds SpirvCompilation.SetDefaultTargetVersionForCrossCompileTarget(CrossCompileTarget, uint version) to allow caller to set global default to avoid having to pass versions into every Compile call

While working on the PBR sample, I found that I ran into compilation errors due to the default MSL version being too low. This change allows me to set the MSL version as seen here.

[TechPizzaDev]

Just wanted to chime in and say that veldrid-spirv translation generates shaders with different versions and causes Error linking GL program: L0001 Shader languages do not match on my Android. This PR may be a "solution" to this problem. My current solution is to just replace the lang version in the generated files to the highest version generated.