Add GPUSampler to WebGPU implementation

Add dom_struct and webidl for GPUSampler, implement GPUDevice.createSampler() method.

components/script/script_thread.rs

@@ -2014,6 +2014,7 @@ impl ScriptThread {
                 self.gpu_id_hub.lock().kill_bind_group_layout_id(id)
             },
             WebGPUMsg::FreeCommandBuffer(id) => self.gpu_id_hub.lock().kill_command_buffer_id(id),
+            WebGPUMsg::FreeSampler(id) => self.gpu_id_hub.lock().kill_sampler_id(id),
             WebGPUMsg::FreeShaderModule(id) => self.gpu_id_hub.lock().kill_shader_module_id(id),
             _ => {},
         }