Allow setting span in custom/extern completers (nushell#15888)

# Description

Closes nushell#15863. This PR allows
custom and extern completers to set the `span` field for the
`Suggestion`s they provide.

The motivation for this is mainly making it easier to make completers
for Zoxide.

# Release notes summary

Custom completers and external completers will be able to choose which
part of the buffer to replace. This is useful if you also want to
replace one of the previous arguments to a command, or if you only want
to replace part of an expression.

# Tests + Formatting

Didn't add any tests, but did manually try the following:
```
> def comp [context, pos] { [{value: mwahahaha, description: bar, span: {start: 0, end: $pos}}] }
> def foo [arg: string@comp] {}
```
After creating those definitions, `foo <TAB>` will replace the command
line with `mwahahaha`.

# After Submitting

Needs to be mentioned in the completer docs.

Author: ysthakur

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

@@ -936,6 +936,24 @@ pub fn map_value_completions<'a>(
                             _ => None,
                         };
                     }
+                    "span" => {
+                        if let Value::Record { val: span, .. } = value {
+                            let start = span
+                                .get("start")
+                                .and_then(|val| val.as_int().ok())
+                                .and_then(|x| usize::try_from(x).ok());
+                            let end = span
+                                .get("end")
+                                .and_then(|val| val.as_int().ok())
+                                .and_then(|x| usize::try_from(x).ok());
+                            if let (Some(start), Some(end)) = (start, end) {
+                                suggestion.span = reedline::Span {
+                                    start: start.min(end),
+                                    end,
+                                };
+                            }
+                        }
+                    }
                     _ => (),
                 }
             });

crates/nu-cli/tests/completions/mod.rs

@@ -15,7 +15,7 @@ use nu_protocol::{
 };
 use nu_std::load_standard_library;
 use nu_test_support::fs;
-use reedline::{Completer, Suggestion};
+use reedline::{Completer, Span, Suggestion};
 use rstest::{fixture, rstest};
 use support::{
     completions_helpers::{
@@ -272,6 +272,25 @@ fn customcompletions_no_sort() {
     match_suggestions(&expected, &suggestions);
 }
 
+#[test]
+fn custom_completions_override_span() {
+    let (_, _, mut engine, mut stack) = new_engine();
+    let command = r#"
+        def comp [] { [{ value: blech, span: { start: 1, end: 10 } }] }
+        def my-command [arg: string@comp] {}"#;
+    assert!(support::merge_input(command.as_bytes(), &mut engine, &mut stack).is_ok());
+
+    let mut completer = NuCompleter::new(Arc::new(engine), Arc::new(stack));
+    let completion_str = "my-command b";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    let expected = vec![Suggestion {
+        value: "blech".to_string(),
+        span: Span::new(1, 10),
+        ..Default::default()
+    }];
+    assert_eq!(expected, suggestions);
+}
+
 #[rstest]
 /// Fallback to file completions if custom completer returns null
 #[case::fallback(r#"
@@ -797,6 +816,20 @@ fn external_completer_fallback() {
     match_suggestions(&expected, &suggestions);
 }
 
+#[test]
+fn external_completer_override_span() {
+    let block = "{|spans| [{ value: blech, span: { start: 1, end: 10 } }]}";
+    let input = "foo b";
+
+    let suggestions = run_external_completion(block, input);
+    let expected = vec![Suggestion {
+        value: "blech".to_string(),
+        span: Span::new(1, 10),
+        ..Default::default()
+    }];
+    assert_eq!(expected, suggestions);
+}
+
 /// Fallback to external completions for flags of `sudo`
 #[test]
 fn external_completer_sudo() {