♻️🍱💩👷 crates/vanth, project: Refactor entity system and setup; migrate to Nix flakes

- Add `.cargo/config.toml` with target directory and rustflags configuration.
- Remove devenv files and set up Nix flake configuration with `flake.nix` and `flake.lock`.
- Add dependencies on `blake3`, `digest`, and hashing-related crates for content hashing functionality.
- Implement `EntityId` as hashed representation and `ContentHash` component using Blake3.
- Add `hashing_serializer` module for serializing and hashing components.
- Remove unused `parser` module and restructure lib.rs with new `Reference` type and storage interfaces.
- Add test files for `store` and `reference` functionality with TODOs for implementation.
- Introduce `VanthPlugin` skeleton and entity save/load interfaces with placeholder implementations.
- Add `Reference` type with retrieval state machine and async handling stubs.

Cargo.toml

@@ -9,5 +9,8 @@ edition = "2024"
 [workspace.dependencies]
 bevy_app = "0.16.1"
 bevy_ecs = "0.16.1"
+bitcode = "0.6.6"
 serde = { version = "1.0.219", features = ["derive"] }
 serde_json = "1.0.140"
+digest = "0.10.7"
+blake3 = { version = "1.8.2", features = ["traits-preview"] }