Add editable superflat terrain layer selector on world creation screen

When creating a new world in Bedrock Edition, add a simple editor to customize superflat layers. Players can switch between built-in flat presets and make their own thick dirt/stone flat terrain easily, without add-ons or external tools. For example, we should be able to set dozens of dirt layers to make very thick flat dirt ground directly on world creation screen. This helps builders, redstone players and map makers a lot