Improve map generation and representation, develop the tile system.

[PabloMansanet]

At the moment, the tile system in the demo is very basic.

Expand ASCII map parsing

• Add more tiles and their associated ASCII characters to represent the current MVP.
• Add spawn point markers to the ASCII representation.
• Add a bevy system to choose the maps to be changed between rounds.

Map representation

Currently, the map is an immutable, Arc wrapped structure to deal with the limitations of wasm callbacks, and game objects such as the character and death marker don't reside in the map at all, but are simply rendered on top of it. This wasm limitation will be gone, so we don't have to worry about Arc to share references with the wasm instances.

Some thoughts:

• To be idiomatic in an ECS system we likely don't want the map to be a big struct that literally contains the game elements. Instead renderable elements will be represented by entities containing some kind of WorldLocation component.
• When passing the "surroundings" to a character's wasm module, we'll probably have to change abstractions a bit and pass Tile enums to the character which may contain things in them, e.g. Tile::Wall, Tile::Floor(None), Tile::Floor(Some(Bomb(countdown: 2))). So we need a bevy system/function that iterates over all things with WorldLocation and works out the configuration of these enums to pass to the character.

Helpful crates

There are crates out there designed for this sort of situation that may help us, bevy_tilemap is the original one I looked at, though I see bevy_ecs_tilemap helps representing tiles in a more ECS idiomatic way so it may be worth looking into as well.

[PabloMansanet]

Implemented by #17 . I looked at both bevy_tilemap and bevy_ecs_tilemap and I didn't love either of them, looks like the biggest benefit in going for them would be performance, not ergonomics, and that's not a priority right now so I decided to go with something custom instead.