Introduce completion abstractions to nushell. (#2198)

* Introduce completion abstractions to nushell.

Currently, we rely on rustyline's completion structures. By abstracting this
away, we are more flexible to introduce someone elses completion engine, or our
own.

* Update value_shell.rs

Co-authored-by: Jonathan Turner <jonathandturner@users.noreply.github.com>

crates/nu-cli/src/lib.rs

@@ -15,6 +15,7 @@ extern crate quickcheck_macros;
 
 mod cli;
 mod commands;
+mod completion;
 mod context;
 pub mod data;
 mod deserializer;