A ~~simpler~~ very similar version of https://github.com/flutter/engine/pull/43854 * Makes it harder for users to accidentally remove default configuration values, while still allowing them to do so if needed (configuration is now overridden with a subset of values, rather than passing a full configuration object). * Moves `merge` from the configuration object and into the override method. * Removes a test-only configuration option: * `window._flutter_canvaskit_variant_for_test_only` [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style