Implement a basic spawn, despawn logic for bombs and flame.

[skywhale]

Spawning & despawning bombs

I added a new BombPlugin to control the spawning and despawning of the bombs and the flames. A bomb has a timer for the fuse, and when it expires, flame replaces the bomb and extends up to 2 blocks in each direction until it hits a wall or a crate. Flame despawns after a period.

Despawning crates

Crates on fire despawn immediately. This is currently handled by a system in GameMapPlugin. This is where I'm still confused trying to learn the idiomatic ways of handling entity interactions. Doing it in GameMapPlugin decouples it from the bomb logic, but it feels suboptimal to repeat the search for overlapping Flame and Object.

Add sound effect assets

Added some sound effects from Storyblocks. I'll keep adding more assets, but please let me know if you want some specific audio assets to be added. I have a paid subscription.

What's not working yet

• Sprite animation for the flame
• Visualization of the crates on fire
• Players should despawn when they touch the flame

[skywhale]

If the new commit looks good to you, please feel free to merge this, so you can rebase your WIP PR on the master branch. Thank you @PabloMansanet