Add first empty slot in inventory expression

[sovdeeth]

Problem

There was no easy way to detect the first empty slot in an inventory without looping over all the slots manually. Bukkit provides a easy firstEmpty() expression that avoids this nonsense, so there was room for improvement.

Solution

Adds an expression that uses firstEmpty() to get the first empty slot in an inventory.
[the] first empty slot in %inventories%, along with the typical property expression patterns.

Testing Completed

ExprFirstEmptySlot.sk tests basic inventory behavior, some manual testing was done to confirm it doesn't return armor slots for players.

Supporting Information

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Completes: none
Related: none

[jurajplayercze]

Is the optional [s] there on purpose? I mean, it always returns the first empty slot anyway
patterns.add("[the] first empty slot[s] in %inventories%");

[erenkarakal]

Is the optional [s] there on purpose? I mean, it always returns the first empty slot anyway patterns.add("[the] first empty slot[s] in %inventories%");

it's because the pattern supports multiple inventories

[jurajplayercze]

it's because the pattern supports multiple inventories

Yeah silly me didn't notice the plural