Add ability to travel through portals from level to level

[razcore-rad]

from Discord:

The way it works is: if there's no next_level_portal_name set it uses the first checkpoint under Checkpoints node, to position the player in the new level, this has the benefit that it immediately visits the checkpoint so it gets saved for the dying state and if next_level_portal_name is set then it uses that specific portal to place the player in the new scene/level

• had to add a Player.has_traveled bool "check" which is used when traveling from portal to portal between levels so that _on_body_entered doesn't trigger the level load on first pass
• also removed unused Exit.* files from under src/Level and renamed src/Objects/Exit.* to Portal.*
• removed Checkpoints node under Game & moved visited_checkpoints-related mechanics inside Game.gd
• changed visited_checkpoints from simple Array to Dictionary to track which checkpoints are from what level and to "re-apply" is_visited checks when triggering level loading (in LevelLoader.gd)

closes #145

[NathanLovato]

Seems good to me, is it good to merge? I'd test it in practice and make more changes later

[razcore-rad]

Yeah, good to merge, it isn't WIP, it's just that I think we can possibly improve the system, it seems a bit more complex than it should, but I think it gives a good test bed for later updates

[NathanLovato]

Got it, I'll merge and we can review it later!