XP system and bar

[Turtyo]

Description

This PR adds the XP system in the game, allowing the player to gain buffs at the start of each combat depending on the amount of XP they have. They gain this XP by soothing enemies.

Related issue(s)

Closes #55

List of changes

• Add a manager to check player XP
• Add a list of buffs to be applied to the player at the start of combat
• Add a bar to show current XP and XP to next level
• Fix an issue with buff with infinite duration being incorrectly removed from the list of status of entities on combat turn start

Tests

• Xp increase
• Buff list
• Next and previous xp level update

Additional notes

Might add some more tests. There is also a slight visual problem when entering a new combat, the XP bar will "jump" to its correct value. This is because the tween will put it there with a time value of 0, but it's still visible.

A solution would be to calculate the correct value to assign it directly in the ready function.

Another solution is to keep the value stored in the XP manager when the XP bar gets killed, so it can pull the correct value from the XP manager next time it's recreated.

Solution one might imply some code duplication but I could make a function. Solution two feels unnecessary because the XP manager should not have to store something related to the visual state of another object imo. I'm open to suggestions on this.