Allow enchanting tables to enchant any item when plugins prompt them to.

Spigot port of the following CB PR: https://github.com/Bukkit/CraftBukkit/pull/1156

The CB PR provides testing materials PR, and this port is identical. I have not tested it myself because the author of the CB PR already made assurances that it worked and did not interfere with vanilla enchanting that has not been modified by plugins.

This PR can be used to allow enchanting tables that have been set selectable enchanting levels in the preparation event to fire the actual enchant event, which currently does not happen with items that can't be enchanted in vanilla.