lufia2ac: ability to swap party members mid-run and option to gain EXP while inactive

[el-u]

What is this fixing or adding?

In AP, the option shuffle_party_members can be used to turn party members into a multiworld unlockable. Custom items were added to the game that enable the player to add newly unlocked members to the party even while a cave dive is already in progress (as opposed to the vanilla behavior where all characters are available form the start, but you can only change your party in town).

Until now, there were 2 shortcomings associated with this feature:

• There was no way to remove party members during a run. So if you used whatever characters you unlocked first to fully fill up your party, but then were later sent a more desirable character, the only way to get them in would be to abort the current run and go back to town to reform your party.
  With this PR, the custom item that adds a character to the party can now simply be used again to remove them, allowing players to swap any unlocked characters in and out at will.
• Only characters in the current party could gain EXP. So if you added a newly unlocked character when you had progressed deep into the cave already, they would join at your chosen party_starting_level, potentially being far behind in power compared to any characters that had been present all the way from the start of the run.
  With this PR, the new option inactive_exp_gain allows characters to gain EXP even while they are not present in the current party. Players can choose between full, half, or no EXP gain for the inactive characters.

How was this tested?

Ludwig tested it.

If this makes graphical changes, please attach screenshots.

[Exempt-Medic]

Tested the changes in a run and everything worked well / as expected. The options and docs changes look fine to me, but I can't say anything about the specifics of the ASM changes