feat: HeightMap mesh builder

[ManevilleF]

Closes #168

Adds a new HeightMapMeshBuilder with its associated heightmap_builder example

[rsrasmu2]

This is excellent! I'm unable to try it on my game as a whole until I'm fully updated to 0.15, but I was able to test it on a subset of my terrain generation and it's working quite well! I have a few thoughts and questions, but none of them are blocking.

1. HeightMapMeshBuilder uses a std::HashMap while Bevy seems to prefer hashbrown::HashMap. It may be worth switching over for easier integration with Bevy.
2. It would be nice if HeightMapMeshBuilder had a constructor that accepted a HexagonalMap<f32>.
3. It would also be helpful if HeightMapMeshBuilder allowed you to set the UVs and Options for individual tops and sides, rather than having a singular FaceOptions and UVOptions apply to all hexes in the heighmap.

Thanks for working on this feature, it's going to simplify a good chunk of my codebase!

[ManevilleF]

1. `HeightMapMeshBuilder` uses a `std::HashMap` while Bevy seems to prefer `hashbrown::HashMap`. It may be worth switching over for easier integration with Bevy.

I'll see what I can do to improve this

2. It would be nice if `HeightMapMeshBuilder` had a constructor that accepted a `HexagonalMap<f32>`.

I guess I could accept a Into<HashMap> or force a dense map like HexagonalMap, and not allow any holes but seems restrictive

3. It would also be helpful if `HeightMapMeshBuilder` allowed you to set the UVs and Options for individual tops and sides, rather than having a singular `FaceOptions` and `UVOptions` apply to all hexes in the heighmap.

Could you give me an example on how that would look like API wise ? A side could be identified by a coordinate and a direction

[rsrasmu2]

I guess I could accept a Into<HashMap> or force a dense map like HexagonalMap, and not allow any holes but seems restrictive

Yeah, no need to do something that would not allow for holes. I just use Hexx's storage types all over my project and it would remove a tiny bit of boilerplate if I could use them with HeightMapMeshBuilder directly.

Could you give me an example on how that would look like API wise ? A side could be identified by a coordinate and a direction

Perhaps mirroring ColumnMeshBuilder with something like

pub const fn with_hex_cap_options(mut self, options: HashMap<Hex, Option<FaceOptions>>) -> Self
pub const fn with_hex_side_options(mut self, options: HashMap<Hex, Option<FaceOptions>>) -> Self
pub const fn with_hex_multi_sides_options(mut self, options: HashMap<Hex, [Option<FaceOptions>;6]>) -> Self

Any hexes that have not had their FaceOptions explicitly set by the above functions use HeightMapMeshBuilder's top_face_options and side_options fields.

[ManevilleF]

@rsrasmu2 Investigating a bit and I noticed that the storages need to be upgraded to match hashmap behavior, which is a bit more work than I intend for this PR.
I also need to think about the API for the side management, which is is complex due to the fact that we only generate 1 side per pair of neighboring hexagons.

I'm going to merge the PR as it is standing, and we can iterate on those suggestions before the release.

Could you convert your comments/suggestions to an issue ?