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/lib.rs

@@ -9,11 +9,9 @@ mod lex;
 mod parse;
 mod scope;
 mod shapes;
-mod signature;
 
 pub use lex::lexer::{lex, parse_block};
 pub use lex::tokens::{LiteBlock, LiteCommand, LiteGroup, LitePipeline};
 pub use parse::{classify_block, garbage, parse, parse_full_column_path, parse_math_expression};
 pub use scope::ParserScope;
 pub use shapes::shapes;
-pub use signature::{Signature, SignatureRegistry};