Add OptionEntry for classic PvP Mode

[obligaron]

Classic PvP Mode currently contains two features

• Reintroduce diagonal/sidewalk missile immunity (only for player vs player; monster aren't affected)
• Disables mouse-hold feature (only for player attacks; wanted feature to make pvp more difficulty)

I know this is a controversial topic, cause:

• Should we reintroduce a bug?
• Should we add more options and complexity only for pvp?

That's why I tried to introduce only one option that enables all wanted behaviors.

Currently only a draft, cause:

• Feedback and testing from pvp players is needed (please test this pr. I'm no pvp player 😉)
• We need a consent in the community if we want to go this way. But a pr that show the size of the needed changes should help to argue better about this topic.

Additional note: I have no problem, if we come to the conclusion that we don't want to merge this pr.

This pr now also includes arena support (second commit). Currently accessible by farnham.

Example Video

https://user-images.githubusercontent.com/25415264/174501014-399a67a4-8e88-4c34-bc17-7dbdffddfd44.mp4

[julealgon]

For modding and scalability reasons, I think it would be best if each individual behavior had its own toggle internally (instead of a single "classic PVP" toggle).

Having said that, I know this is not common, but we could make it so that the UI still presents this as a single "Classic PVP" option in the settings page, but then this toggles both options internally. This could be a way to avoid exposing complexity to folks playing the game.

If people then opt to toggle each behavior individually in the config file, the UI could just disable the setting (so that it doesn't need to decide "what to do" in case there is a mix of enabled and disabled flags all part of the "classic pvp" thing).

We discussed this at some point in a different context, where we would have "named profiles" that would then have predefined settings flags.