Add NU config to allow user be able to turn off external completion (#5773)

* 06-07-wsl

* 06-07-linux-issue-with-delete-input

* 06-08-2023

* 06-08-Linux

* commit for merge

* Fix unit test

* format

* clean code

* Add flag to turn off external completion

* change env var to config

* Fix comment

Co-authored-by: Frank Zhang <v-frankz@microsoft.com>

crates/nu-cli/src/completions/command_completions.rs

@@ -199,12 +199,19 @@ impl Completer for CommandCompletion {
             return subcommands;
         }
 
+        let config = working_set.get_config();
         let commands = if matches!(self.flat_shape, nu_parser::FlatShape::External)
             || matches!(self.flat_shape, nu_parser::FlatShape::InternalCall)
             || ((span.end - span.start) == 0)
         {
             // we're in a gap or at a command
-            self.complete_commands(working_set, span, offset, true, options.match_algorithm)
+            self.complete_commands(
+                working_set,
+                span,
+                offset,
+                config.enable_external_completion,
+                options.match_algorithm,
+            )
         } else {
             vec![]
         };