fix(completion): completions.external.enable config option not respected (#15443)

Fixes #15441 

# Description

Actually I made a small change to the original behavior:

```
^foo<tab>
```
will still show external commands, regardless of whether it's enabled or
not. I think that's the only thing people want to see when they press
tab with a `^` prefix.

# User-Facing Changes

# Tests + Formatting

+1

# After Submitting

Should I document that minor behavior change somewhere in GitHub.io?

---------

Co-authored-by: Yash Thakur <45539777+ysthakur@users.noreply.github.com>

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

@@ -486,9 +486,10 @@ impl NuCompleter {
         externals: bool,
         strip: bool,
     ) -> Vec<SemanticSuggestion> {
+        let config = self.engine_state.get_config();
         let mut command_completions = CommandCompletion {
             internals,
-            externals,
+            externals: !internals || (externals && config.completions.external.enable),
         };
         let (new_span, prefix) = strip_placeholder_if_any(working_set, &span, strip);
         let ctx = Context::new(working_set, new_span, prefix, offset);