Prepare ItemTypes for detect and translate phases in connectors

Is this what you had in mind @simonster over at https://github.com/zotero/translators/commit/51342c4a886b3066b02b389faa0f88a0538e3b2d#commitcomment-2639758 ?

Hopefully I understood how ItemType initialization works. I'm not sure if we need to run _initItemTypes in for translate if we have already done so for detect. If I understand this correctly, Zotero.ItemTypes should always hang around until translate is called. And I don't see hoe translate can be called without detect being called first.