Make nightly features optional

Currently andromeda specifies nova_vm features in workspace `Cargo.toml`. In which the `proposals` feature (`proposal-float16array` specifically) brings in a nightly Rust feature `f16`. <https://github.com/tryandromeda/andromeda/blob/4a00731a1a6d85af22bd2bab2544aed5777bde81/Cargo.toml#L43-L47> This is okay for andromeda cli itself, but brings difficulties to embedding and distribution. Since Rust features is [additive](https://doc.rust-lang.org/cargo/reference/features.html#feature-unification), downstream users cannot remove these features, forcing them to use a nightly Rust, even if they only use `andromeda-core` and `andromeda-runtime`. A simple fix can be to alternatively define `features` in `cli` package instead of the workspace one.