Core+WASM: Provide a public and fully-fledged CityBuilder to the WASM interface

[pierreyoda]

Following the builder pattern, provide a new CityBuilder facility accessible from the WASM interface.

Proposed interface

let mut city_generator = CityGeneratorBuilder::new()
    .set_map_generator_options(/* ... */)
    .set_simulation_rng_seed(123456)
    .build()?;
let map1 = city_generator.build_random_map(1234)?;
let map2 = city_generator.build_random_map(4567)?;
let city = city_generator.generate(GameLevelDifficulty::Normal, map1)?;