Initial implementation of GPUCommandEncoder

Added WebIDL bindings for `GPUCommandEncoder`, `GPUCommandBuffer`, `GPUComputePassEncoder`, `GPUProgrammablePassEncoder`.
Implemented the `beginComputePass`, `copyBufferToBuffer` and `finish` functions of `GPUCommandEncoder`.
Implemented the `createCommandEncoder` function of `GPUDevice`.

components/script/dom/webidls/GPUCommandEncoder.webidl

@@ -0,0 +1,45 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+// https://gpuweb.github.io/gpuweb/#gpucommandencoder
+[Exposed=(Window, DedicatedWorker), Serializable, Pref="dom.webgpu.enabled"]
+interface GPUCommandEncoder {
+    // GPURenderPassEncoder beginRenderPass(GPURenderPassDescriptor descriptor);
+    GPUComputePassEncoder beginComputePass(optional GPUComputePassDescriptor descriptor = {});
+
+    void copyBufferToBuffer(
+        GPUBuffer source,
+        GPUBufferSize sourceOffset,
+        GPUBuffer destination,
+        GPUBufferSize destinationOffset,
+        GPUBufferSize size);
+
+    // void copyBufferToTexture(
+    //     GPUBufferCopyView source,
+    //     GPUTextureCopyView destination,
+    //     GPUExtent3D copySize);
+
+    // void copyTextureToBuffer(
+    //     GPUTextureCopyView source,
+    //     GPUBufferCopyView destination,
+    //     GPUExtent3D copySize);
+
+    // void copyTextureToTexture(
+    //     GPUTextureCopyView source,
+    //     GPUTextureCopyView destination,
+    //     GPUExtent3D copySize);
+
+    // void pushDebugGroup(DOMString groupLabel);
+    // void popDebugGroup();
+    // void insertDebugMarker(DOMString markerLabel);
+
+    GPUCommandBuffer finish(optional GPUCommandBufferDescriptor descriptor = {});
+};
+GPUCommandEncoder includes GPUObjectBase;
+
+dictionary GPUComputePassDescriptor : GPUObjectDescriptorBase {
+};
+
+dictionary GPUCommandBufferDescriptor : GPUObjectDescriptorBase {
+};