refactor(completion, parser): move custom_completion info from Expression to Signature (#15613)

Restricts custom completion from universal to internal arguments only.

Pros:
1. Less memory
2. More flexible for later customizations, e.g. #14923 

Cons:
1. limited customization capabilities, but at least covers all currently
existing features in nushell.

# Description

Mostly vibe coded by [Zed AI](https://zed.dev/ai) with a single prompt.
LGTM, but I'm not so sure @ysthakur 

# User-Facing Changes

Hopefully none.

# Tests + Formatting

+3

# After Submitting

---------

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

crates/nu-protocol/tests/test_signature.rs

@@ -45,6 +45,7 @@ fn test_signature_chained() {
             shape: SyntaxShape::String,
             var_id: None,
             default_value: None,
+            custom_completion: None,
         })
     );
     assert_eq!(
@@ -55,6 +56,7 @@ fn test_signature_chained() {
             shape: SyntaxShape::String,
             var_id: None,
             default_value: None,
+            custom_completion: None,
         })
     );
     assert_eq!(
@@ -65,6 +67,7 @@ fn test_signature_chained() {
             shape: SyntaxShape::String,
             var_id: None,
             default_value: None,
+            custom_completion: None,
         })
     );
 
@@ -78,6 +81,7 @@ fn test_signature_chained() {
             desc: "required named description".to_string(),
             var_id: None,
             default_value: None,
+            custom_completion: None,
         })
     );
 
@@ -91,6 +95,7 @@ fn test_signature_chained() {
             desc: "required named description".to_string(),
             var_id: None,
             default_value: None,
+            custom_completion: None,
         })
     );
 }