Engage by ID

This pull request adds an engage by ID feature to the GearSwap core, allowing players to engage alts with send without needing to rely on, and occasionally be impacted by, the potential unreliability of <bt> and /assist. It's also just plain faster.

This would make GearSwap not only allow players to cast spells by mob ID, but also engage by mob ID.

Safeguards:

This feature includes the following safeguards:

1. It only functions while the player is idle or engaged (can't be dead, on a chocobo, fishing, etc)
2. It does nothing if given an invalid ID (cannot find, no index, valid_target is false)
3. It does not function while charmed
4. Does not engage/switch-target to a mob that the player is already engaged to
5. Will not produce a disengagement packet if the player is already disengaged
6. It will stop itself from building a packet if it is unable to assemble all the required information
7. It won't intercept the vanilla attack commands if they lack an ID (/attack, /attack on, /attack off, /attackoff)
8. Interestingly, it filters out the vanilla "Attack" button's "/attack 12345678" text input, by utilizing the ffxi variable

Commands:

NOTE: Utilize a method, such as send's <tid> or <laststid> to obtain valid mob ids for these commands.

/attack 12345678 - toggles engagement on the given mob (only disengages if the given mob is already engaged)
/attack on 12345678 - engages or switches target to the given mob
/attack off 12345678 - disengages from the given mob only if it is already engaged by the current player
/attackoff 12345678 - disengages from the given mob only if it is already engaged by the current player
/attackon 12345678 - omitted, SE did not include this as a base command

How to Use:

1. [Direct Targeting] Create a/multiple alias(es) such as "alias attackonall send @all input /attack on", then use "//attackonall <tid>" ingame.
2. [Subtargeting] Create a macro such as "/target <stnpc>[newline]/con send @all input /attack on <laststid>"

Use Cases:

1. Use "//send @all /attack on <tid>" to engage all alts on the given mob
2. In a multi-mob engagement scenario, if mob 34567890 is being slept, use "//send @all input /attackoff <tid>" while targeting said mob to disengage all alts from the given mob without disturbing alts that are engaged to a different mob
3. Use "//send charname /attack on <tid>" to have that specific alt engage or switch-target to the given mob

Behavior

You can expect much of the same behavior as when your client naturally generates these packets, but without your alts needing to do the preliminary targeting any longer.
Engage: Your client, all at once, automatically targets said mob (new), locks on, and engages.
Switch Target: Your client automatically switches to said mob (new), keeping your current targeting status (locked/unlocked).
Disengage: Your client disengages from said mob. (Does nothing if alt was engaged to a different mob.)

Noteworthy: While this feature doesn't check if the mob is claimed by another party/alliance, neither does the vanilla "Attack" command button. Add a logger to outgoing chunks and then try to attack a claimed enemy via the vanilla command button; you will notice that vanilla FFXI sends out an engage packet that the server rejects! It is for that reason that we didn't (potentially needlessly) complicate this aspect of the feature in comparison to vanilla FFXI's behavior in this same aspect.