Support NBT path format

[rj00a]

Describe the problem related to your feature request.

There is currently no support for NBT path format in valence_nbt.

NBT path format support is a prerequisite for a complete command parser and valence_command crate.

What solution would you like?

Add a new path module and feature flag to valence_nbt.

Paths get parsed into a structure that looks something like this:

pub struct NbtPath<S = String> {
    string: S,
    nodes: Vec<NbtPathNode>,
}

pub enum NbtPathNode {
    RootCompoundTag { ... },
    NamedTag { ... },
    NamedCompoundTag { ... },
    ...
}

Once the NbtPath is parsed, lookups can be performed on &Compound or &mut Compound values.

I would also like to see some unit tests and assurance that our implementation closely matches vanilla Minecraft's behavior.

What alternative(s) have you considered?

This is less of an alternative and more of an addition to the above solution, but we could add methods on Compound (named path, path_mut?) similar to serde_json's pointer and pointer_mut methods. The implementation should not parse to an intermediate NbtPath to avoid the associated overhead.

Additional context

#332

[fdfdjfk3]

i'd like to try working on this if nobody is doing it already 👍

[dyc3]

Sounds good, I'll assign you. Looking forward to your PR!