Enable WebGPU backend for Emscripten builds

[j8asic]

Added support for compiling slang-rhi using Emscripten (WASM), while enabling the WebGPU backend and handling dependencies for the web platform:

• Added logic to detect emscripten
• enable SLANG_RHI_HAS_WGPU and disable incompatible native APIs when building for Emscripten
• Skipped fetching the native Dawn binary package when building for Emscripten
• target_compile_options use modern --use-port=emdawnwebgpu for Emscripten builds
• core rhi code switches between dawn/emscripten functionality

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