Split nu-cli into nu-cli/nu-engine (#2898)

We split off the evaluation engine part of nu-cli into its own crate. This helps improve build times for nu-cli by 17% in my tests. It also helps us see a bit better what's the core engine portion vs the part specific to the interactive CLI piece.

There's more than can be done here, but I think it's a good start in the right direction.

crates/nu-cli/src/line_editor.rs

@@ -1,4 +1,4 @@
-use crate::evaluation_context::EvaluationContext;
+use nu_engine::EvaluationContext;
 use std::error::Error;
 
 #[allow(unused_imports)]