Fix indentation of schema file and fix several small issues.

__Changes:__

* Use only tabs to indent the file instead of a mixture of tabs and spaces.
* Standardize several data type & element base names.
* Use a custom boolean type for attributes accepting true/false.
* Fix/add the include element’s src attribute.
* Remove the slot attribute from helmet, chestplate, leggings & boots.
* Restrict the item slot attribute to only the actual slot numbers.
* Circle was spelled cirlce.
* Add `damage resistance` to potion types.