[HLSL][SPIRV] Add a `KHR` special option to `spirv-ext` CLI option

[s-perron]

In DXC, there is an options to limit the extension to just the KHR options: -fspv-extension=KHR. We would like to add the same to spirv-ext to make it easier to be backwards compatible with DXC.

This can be implemented using a special value in SPIRVExtensionsParser::parse like "all". Then it can traverse the SPIRVExtensionMap to add all entries that start with "SPV_KHR".