[WebGPU] Update and Correct example and backend

[manuel5975p]

• Get both the WebGPU backend and its example up to date with the current (standardized) webgpu.h header.
  • API change for wgpuInstanceRequestAdapter
  • API change for wgpuAdapterRequestDevice
  • Some renamings, i.e. WGPUEmscriptenSurfaceSourceCanvasHTMLSelector and its sType
• Remove dependencies on the C++ header webgpu_cpp.h as it's nonstandard and doesn't provide much benefit here

The current implementation has been tested against webgpu.h generated by chromium/7281

In the example, a glfw callback is added to correctly deal with window resizes.

However, here's some parts of this PR that are still unrefined:

• A lot of places use #ifdef switching depending on the backend to choose a code path. This PR keeps it that way, however this should maybe be removed entirely? The only purpose of this is to deal with outdated headers of some implementations.
• Similarly, compiling for web requires the WEBGPU_BACKEND_EMDAWNWEBGPU flag and the IMGUI_IMPL_WEBGPU_BACKEND_DAWN at the same time, which only works by commenting out an #error directive.

[ocornut]

Thanks! Linking to #8674 #8567 #8381

FYI I am struggling to take on action with any of those since they are difficult to test :(

Also, any PR which exhibit the fact that the submitting person has not carefully reviewed every line of the PR is also quite unfortunately immediately taking a huge hit in term of trust. It's difficult for me to consider reviewing a PR if I feel that I cannot trust the submitter. It's 100% certain there are useful stuff in there, which I can use as reference at minimum, but if I can spot 5 mistakes in one minute of reviewing the whole thing begins with a huge hit on trust.

[ocornut]

As part of trying to untangle #8381 (funnily one PR is #8381 and the other is #8831)
I have extracted a ImGui_ImplGLFW_CreateWGPUSurface() helper function into proposed 63ef7e6., which we can refine based on your equivalent glfwCreateWindowWGPUSurface() function.

[ocornut]

Most of this is now solved by #8381 now.

Leftover that I think we could borrow from this PR:

(1) Add support for IMGUI_IMPL_WEBGPU_BACKEND_WGVK. I'll leave it to any WGVK user to resume work from current master but I suppose it may be trivial now.

(2) This block

#  ifdef _GLFW_X11
#    define GLFW_EXPOSE_NATIVE_X11
#  endif
#  ifdef _GLFW_WAYLAND
#    define GLFW_EXPOSE_NATIVE_WAYLAND
#  endif
#  ifdef _GLFW_COCOA
#    define GLFW_EXPOSE_NATIVE_COCOA
#  endif
#  ifdef _GLFW_WIN32
#    define GLFW_EXPOSE_NATIVE_WIN32
#  endif

Is something we could investigate/borrow in our backend, aka using the _GLFW_WAYLAND etc exposed by glfw.h.