Add support for defining known externals with their own custom completions (#4425)

* WIP for known externals

* Now completions can work from scripts

* Add support for definiing externs

* finish cleaning up old proof-of-concept

crates/nu-protocol/src/engine/command.rs

@@ -41,6 +41,11 @@ pub trait Command: Send + Sync + CommandClone {
         true
     }
 
+    // This is a signature for a known external command
+    fn is_known_external(&self) -> bool {
+        false
+    }
+
     // Is a sub command
     fn is_sub(&self) -> bool {
         self.name().contains(' ')