feat(completion): stdlib virtual path completion & exportable completion (#15270)

# Description

More completions for `use` command.

~Also optimizes the span fix of #15238 to allow changing the text after
the cursor.~

# User-Facing Changes

<img width="299" alt="image"
src="https://github.com/user-attachments/assets/a5c45f46-40e4-4c50-9408-7b147ed11dc4"
/>

<img width="383" alt="image"
src="https://github.com/user-attachments/assets/fbeec173-511e-4c72-8995-bc1caa3ef0d3"
/>


# Tests + Formatting

+3

# After Submitting

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

@@ -11,6 +11,7 @@ use nu_engine::eval_block;
 use nu_parser::parse;
 use nu_path::expand_tilde;
 use nu_protocol::{debugger::WithoutDebug, engine::StateWorkingSet, Config, PipelineData};
+use nu_std::load_standard_library;
 use reedline::{Completer, Suggestion};
 use rstest::{fixture, rstest};
 use support::{
@@ -513,7 +514,7 @@ fn dotnu_completions() {
 
     match_suggestions(&vec!["sub.nu`"], &suggestions);
 
-    let expected = vec![
+    let mut expected = vec![
         "asdf.nu",
         "bar.nu",
         "bat.nu",
@@ -546,6 +547,8 @@ fn dotnu_completions() {
     match_suggestions(&expected, &suggestions);
 
     // Test use completion
+    expected.push("std");
+    expected.push("std-rfc");
     let completion_str = "use ";
     let suggestions = completer.complete(completion_str, completion_str.len());
 
@@ -577,6 +580,65 @@ fn dotnu_completions() {
     match_dir_content_for_dotnu(dir_content, &suggestions);
 }
 
+#[test]
+fn dotnu_stdlib_completions() {
+    let (_, _, mut engine, stack) = new_dotnu_engine();
+    assert!(load_standard_library(&mut engine).is_ok());
+    let mut completer = NuCompleter::new(Arc::new(engine), Arc::new(stack));
+
+    let completion_str = "export use std/ass";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&vec!["assert"], &suggestions);
+
+    let completion_str = "use `std-rfc/cli";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&vec!["clip"], &suggestions);
+
+    let completion_str = "use \"std";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&vec!["\"std", "\"std-rfc"], &suggestions);
+
+    let completion_str = "overlay use \'std-rfc/cli";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&vec!["clip"], &suggestions);
+}
+
+#[test]
+fn exportable_completions() {
+    let (_, _, mut engine, mut stack) = new_dotnu_engine();
+    let code = r#"export module "🤔🐘" {
+        export const foo = "🤔🐘";
+    }"#;
+    assert!(support::merge_input(code.as_bytes(), &mut engine, &mut stack).is_ok());
+    assert!(load_standard_library(&mut engine).is_ok());
+
+    let mut completer = NuCompleter::new(Arc::new(engine), Arc::new(stack));
+
+    let completion_str = "use std null";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&vec!["null-device", "null_device"], &suggestions);
+
+    let completion_str = "export use std/assert eq";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&vec!["equal"], &suggestions);
+
+    let completion_str = "use std/assert \"not eq";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&vec!["'not equal'"], &suggestions);
+
+    let completion_str = "use std-rfc/clip ['prefi";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&vec!["prefix"], &suggestions);
+
+    let completion_str = "use std/math [E, `TAU";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&vec!["TAU"], &suggestions);
+
+    let completion_str = "use 🤔🐘 'foo";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&vec!["foo"], &suggestions);
+}
+
 #[test]
 fn dotnu_completions_const_nu_lib_dirs() {
     let (_, _, engine, stack) = new_dotnu_engine();