Add WebGPU->Vulkan and Vulkan->WebGPU flags in spirv-opt (#2496)

Renames the existing flag '--webgpu-mode' to '--vulkan-to-webgpu' for
the Vulkan->WebGPU operation, and adds a new flag '--webgpu-to-vulkan'
for the WebGPU->Vulkan operation.

Currently '--webgpu-to-vulkan' doesn't have any passes associated with
it yet, but further patches will implement them.

Fixes #2495