Variable completions. (#3666)

In Nu we have variables (E.g. $var-name) and these contain `Value` types.
This means we can bind to variables any structured data and column path syntax
(E.g. `$variable.path.to`) allows flexibility for "querying" said structures.

Here we offer completions for these. For example, in a Nushell session the
variable `$nu` contains environment values among other things. If we wanted to
see in the screen some environment variable (say the var `SHELL`) we do:

```
> echo $nu.env.SHELL
```

with completions we can now do: `echo $nu.env.S[\TAB]` and we get suggestions
that start at the column path `$nu.env` with vars starting with the letter `S`
in this case `SHELL` appears in the suggestions.

crates/nu-protocol/src/lib.rs

@@ -5,6 +5,7 @@ mod call_info;
 pub mod config_path;
 pub mod hir;
 mod maybe_owned;
+mod registry;
 mod return_value;
 mod signature;
 mod syntax_shape;
@@ -18,6 +19,7 @@ pub mod dataframe;
 pub use crate::call_info::{CallInfo, EvaluatedArgs};
 pub use crate::config_path::ConfigPath;
 pub use crate::maybe_owned::MaybeOwned;
+pub use crate::registry::{SignatureRegistry, VariableRegistry};
 pub use crate::return_value::{CommandAction, ReturnSuccess, ReturnValue};
 pub use crate::signature::{NamedType, PositionalType, Signature};
 pub use crate::syntax_shape::SyntaxShape;