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-parser/src/scope.rs

@@ -3,8 +3,6 @@ use nu_source::Spanned;
 use std::{fmt::Debug, sync::Arc};
 
 pub trait ParserScope: Debug {
-    fn get_names(&self) -> Vec<String>;
-
     fn get_signature(&self, name: &str) -> Option<nu_protocol::Signature>;
 
     fn has_signature(&self, name: &str) -> bool;