Servo improves WebGPU support, gets new browser UI

While our WebGPU support is still very much experimental (--pref dom.webgpu.enabled), it now passes over 5000 more tests in the Conformance Test Suite, after an upgrade from wgpu 0.6 (2020) to 0.16 (2023) and the addition of GPUSupportedFeatures. A few WebGPU demos now run too, notably those that don’t require changing the width or height on the fly, such as the Conway’s Game of Life built in Your first WebGPU app.