Adding webgpu-headers submodule. (#6483)

This will allow us to build the WebGPU HAL outside of emscripten.
They are the same as the headers available at
https://github.com/emscripten-core/emscripten/tree/main/system/include/webgpu
diff --git a/.gitmodules b/.gitmodules
index a00f81a..2d8c87a 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -47,3 +47,6 @@
 [submodule "third_party/libyaml"]
 	path = third_party/libyaml
 	url = https://github.com/yaml/libyaml.git
+[submodule "third_party/webgpu-headers"]
+	path = third_party/webgpu-headers
+	url = https://github.com/webgpu-native/webgpu-headers.git
diff --git a/SUBMODULE_VERSIONS.txt b/SUBMODULE_VERSIONS.txt
index 32e59ad..62fb1ca 100644
--- a/SUBMODULE_VERSIONS.txt
+++ b/SUBMODULE_VERSIONS.txt
@@ -14,3 +14,4 @@
 50f7deb1a389bd3785c12fbe0be74128343f11f7 third_party/tracy
 9d10a96f2d57c3c37e167f2e73c9a31ac2e51fa5 third_party/vulkan_headers
 8d4a9e9174a9c6ad6a3a3ae981b915ef13fc12c4 third_party/vulkan_memory_allocator
+c8e0b39f6f6f1edded5c4adf7d46aa4d2a95befe third_party/webgpu-headers
diff --git a/third_party/webgpu-headers b/third_party/webgpu-headers
new file mode 160000
index 0000000..c8e0b39
--- /dev/null
+++ b/third_party/webgpu-headers
@@ -0,0 +1 @@
+Subproject commit c8e0b39f6f6f1edded5c4adf7d46aa4d2a95befe