Does pocketmine fire an event when a tnt block gets ignited? If so, what is its name... Or you have to listen to interact events and check if the player has a lighter in his hand, and the block is a tnt block.... Doing so seems inefficient...
You could listen for the EntitySpawnEvent which will be called as soon as the PrimedTNT entity is created (= after the tnt has been ignited). However this will react to all kinds of TNT ignition like TNT being ignited from another explosion or TNT being ignited from a nearby block burning, not just TNT being primed by a flint and steel by a player.