Create a Minecraft Paper plugin (1.21+) that adds an unbreakable custom sword with special abilities. When a player right-clicks while holding the sword, it shoots a Wither Skull in the direction the player is looking, and when it hits it creates a large explosion that damages nearby entities; this ability has a 10-second cooldown. The sword also has a passive effect: when the player hits another player, there is a 20% chance to apply the Wither effect to the target. Additionally, when the player crouches (shift) and left-clicks, they gain flight for 5 seconds, with a 15-second cooldown. Include a config.yml so server owners can adjust explosion damage, explosion radius, cooldowns, Wither chance and duration, flight duration, and enable/disable particles and sound effects. The sword must be identified using a persistent data tag so it cannot be faked.