feat: prefer exact match when completion mode is prefix (nushell#13302)

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Fixes nushell#13204

# Description
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When the completion mode is set to `prefix`, path completions explicitly
check for and prefer an exact match for a basename instead of longer or
similar names.

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->

Exact match is inactive since there's no trailing slash

```
~/Public/nushell| ls crates/nu-plugin<tab>
crates/nu-plugin/               crates/nu-plugin-core/          crates/nu-plugin-engine/        crates/nu-plugin-protocol/      
crates/nu-plugin-test-support/
```

Exact match is active

```
~/Public/nushell| ls crates/nu-plugin/<tab>
crates/nu-plugin/Cargo.toml  crates/nu-plugin/LICENSE     crates/nu-plugin/README.md   crates/nu-plugin/src/
```

Fuzzy matching persists its behavior

```
~/Public/nushell> $env.config.completions.algorithm = "fuzzy";
~/Public/nushell| ls crates/nu-plugin/car
crates/nu-cmd-plugin/Cargo.toml           crates/nu-plugin/Cargo.toml               crates/nu-plugin-core/Cargo.toml          crates/nu-plugin-engine/Cargo.toml        
crates/nu-plugin-protocol/Cargo.toml      crates/nu-plugin-test-support/Cargo.toml
```

# Tests + Formatting
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# After Submitting
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crates/nu-cli/src/completions/completion_common.rs

@@ -12,20 +12,29 @@ use nu_protocol::{
 use nu_utils::get_ls_colors;
 use std::path::{is_separator, Component, Path, PathBuf, MAIN_SEPARATOR as SEP};
 
-use super::SortBy;
+use super::{MatchAlgorithm, SortBy};
 
 #[derive(Clone, Default)]
 pub struct PathBuiltFromString {
     parts: Vec<String>,
     isdir: bool,
 }
 
-fn complete_rec(
+/// Recursively goes through paths that match a given `partial`.
+/// built: State struct for a valid matching path built so far.
+///
+/// `isdir`: whether the current partial path has a trailing slash.
+/// Parsing a path string into a pathbuf loses that bit of information.
+///
+/// want_directory: Whether we want only directories as completion matches.
+/// Some commands like `cd` can only be run on directories whereas others
+/// like `ls` can be run on regular files as well.
+pub fn complete_rec(
     partial: &[&str],
     built: &PathBuiltFromString,
     cwd: &Path,
     options: &CompletionOptions,
-    dir: bool,
+    want_directory: bool,
     isdir: bool,
 ) -> Vec<PathBuiltFromString> {
     let mut completions = vec![];
@@ -35,7 +44,7 @@ fn complete_rec(
             let mut built = built.clone();
             built.parts.push(base.to_string());
             built.isdir = true;
-            return complete_rec(rest, &built, cwd, options, dir, isdir);
+            return complete_rec(rest, &built, cwd, options, want_directory, isdir);
         }
     }
 
@@ -56,7 +65,7 @@ fn complete_rec(
         built.parts.push(entry_name.clone());
         built.isdir = entry_isdir;
 
-        if !dir || entry_isdir {
+        if !want_directory || entry_isdir {
             entries.push((entry_name, built));
         }
     }
@@ -68,12 +77,29 @@ fn complete_rec(
         match partial.split_first() {
             Some((base, rest)) => {
                 if matches(base, &entry_name, options) {
+                    // We use `isdir` to confirm that the current component has
+                    // at least one next component or a slash.
+                    // Serves as confirmation to ignore longer completions for
+                    // components in between.
                     if !rest.is_empty() || isdir {
-                        completions.extend(complete_rec(rest, &built, cwd, options, dir, isdir));
+                        completions.extend(complete_rec(
+                            rest,
+                            &built,
+                            cwd,
+                            options,
+                            want_directory,
+                            isdir,
+                        ));
                     } else {
                         completions.push(built);
                     }
                 }
+                if entry_name.eq(base)
+                    && matches!(options.match_algorithm, MatchAlgorithm::Prefix)
+                    && isdir
+                {
+                    break;
+                }
             }
             None => {
                 completions.push(built);