add spectator team (and more triggers)

[alexasterisk]

must be ran once:

team add Spectator
team modify Spectator color dark_gray
team modify Spectator prefix "[spec] "
team modify Spectator collisionRule never
team modify Spectator nametagVisibility never

triggers & trigger scoreboards are added automatically by themselves.
these triggers can be used by all people:

trigger become_spectator - toggles if they're a spectator or not, does not work if they have a seat
trigger clear_inventory - completely wipes their inventory excluding items with the persistent tag
trigger clear_books - completely wipes written/writable books excluding books with the persistent tag
trigger toggle_held_item_persistent - toggles the held item's persistent tag