Fixes for emdawnwebgpu

[floooh]

Todo:

• update changelog
• in sokol_app.h: move the code that only depends on webgpu.h of the Emscripten-specific block and instead put it into a platform-agnostic #ifdef SOKOL_WGPU, to make those functions available for other platforms (as preparation for supporting Dawn via sokol_app.h at a later point).
• update sokol_app.h and sokol_gfx.h docs with Emscripten WebGL2 and WebGPU build instructions
• fix sokol-zig build process (note: needs to run embuilder to fetch and build the emdawnwebgpu build first before building the sokol C lib - do we need to inject additional include paths?)
• provide instructions for sokol-d build
• merge Fixes for emdawnwebgpu sokol-samples#170
• merge Emscripten toolchain file: support emdawnwebgpu fips#299
• merge wasm32-emscripten: use emdawnwebgpu for WebGPU support sokol-zig#120
• (later) blocker if we want to enable the Closure pass again: https://issues.chromium.org/issues/424836759