feat: add new onHired and onOtherPlayerHired events for skills

[Darxo]

onHired is an event already present in Vanilla but only in the player.nut and for origins.
Skills however can't react to the hiring of someone.

This PR proposes two new events for skills:

// This event is triggered once for every skill of a player character directly after the onHired event of that player class is triggered
function onHired()

// This event is triggered once for every skill of every other character in the player roster after the onHired event of a player class is triggered
function onOtherPlayerHired( _otherCharacter )

Possible Use Cases

onHired

• Certain backgrounds can apply mood buff/debuff on hiring, depending on what other backgrounds are already present in the player party
• Some traits might act as a wildcard/unrevelaed at first (despite trying out) and only transform into a real trait after hiring

onOtherPlayerHired

• Certain backgrounds might cause a brother to get a positive/negative mood when you hire a certain background
• Certain backgrounds/perks could grant a bonus to every new recruit (e.g. free perk/stats)