Add some kind of API for dig groups

[luk3yx]

Different games sometimes use non-standard dig groups which makes blocks created with other games in mind unbreakable.

• minetest_game use the groups defined in https://github.com/minetest/minetest/blob/master/doc/lua_api.md#known-damage-and-digging-time-defining-groups
• Void uses the crumbly, snappy, and oddly_breakable_by_hand groups for its custom hand
• MineClone uses its own groups (pickaxey, etc)
• Farlands Reloaded uses its own groups (dig_stone, etc)

[wsor4035]

Mineclon*s will be fun given they require a node def param in addition to groups