Add KubeJS API for casings to easily imitate blocks

[Technici4n]

Fixes #689 by introducing the following new syntax for machine casings that imitate an existing block:

MIMachineEvents.registerCasings(e => {
    e.registerBlockImitation('netherrack', 'modern_industrialization:netherrack_machine_casing');
})

e.register('netherrack') only registers the casing, but since we do not automatically generate top/side/bottom copies of the texture anymore for casing parts it leads to a missing model. Either copy the textures manually, add a casing JSON manually, or use registerBlockImitation.