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-cli/src/shell.rs

@@ -27,12 +27,31 @@ impl Helper {
     }
 }
 
+use nu_protocol::{SignatureRegistry, VariableRegistry};
 struct CompletionContext<'a>(&'a EvaluationContext);
 
 impl<'a> nu_completion::CompletionContext for CompletionContext<'a> {
-    fn signature_registry(&self) -> &dyn nu_parser::ParserScope {
+    fn signature_registry(&self) -> &dyn SignatureRegistry {
         &self.0.scope
     }
+
+    fn scope(&self) -> &dyn nu_parser::ParserScope {
+        &self.0.scope
+    }
+
+    fn source(&self) -> &EvaluationContext {
+        self.as_ref()
+    }
+
+    fn variable_registry(&self) -> &dyn VariableRegistry {
+        self.0
+    }
+}
+
+impl<'a> AsRef<EvaluationContext> for CompletionContext<'a> {
+    fn as_ref(&self) -> &EvaluationContext {
+        self.0
+    }
 }
 
 pub struct CompletionSuggestion(nu_completion::Suggestion);