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-command/src/default_context.rs

@@ -38,6 +38,7 @@ pub fn create_default_context(cwd: impl AsRef<Path>) -> EngineState {
             ExportDef,
             ExportDefEnv,
             ExportEnv,
+            Extern,
             For,
             Help,
             Hide,
@@ -358,9 +359,6 @@ pub fn create_default_context(cwd: impl AsRef<Path>) -> EngineState {
         #[cfg(feature = "plugin")]
         bind_command!(Register);
 
-        // This is a WIP proof of concept
-        // bind_command!(ListGitBranches, Git, GitCheckout, Source);
-
         working_set.render()
     };