Modern Beta 1.12.2-1.5.3.0

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Add new custom world preset system, replacing previous way of adding custom presets via the Forge config:
- Save and edit custom presets directly in the world preset GUI.
- Set name, description, settings string, and icon for your custom world preset.
- Custom presets are saved in config/modernbetaforge_presets.json.
Add defaultPresetFilter config setting to set a default preset filter when opening the custom world preset screen.
Remove customPreset config setting.
Fix Modern Beta Skylands preset using incorrect world spawner.