introduce WebGPU via Emdawnwebgpu

[NKID00]

Tracking issue: #5974

This PR introduces Emscripten's native webgpu.h implementation Emdawnwebgpu, and add basic utilities and environments for further WebGPU implementation PRs.

FORCE_FILESYSTEM, INITIAL_MEMORY and EXIT_RUNTIME are linker-only arguments, setting them on compiler does nothing but yields unused argument warnings.

[tencent-adm]

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