[CmdPal] Search PATH starting with ~ / \ (#40887)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request

Love that we have now environment variables expanding! 🚀 
Porting also few small features I implemented in Run Folder plugin a
long time ago and I love:
- https://github.com/microsoft/PowerToys/pull/7711
- https://github.com/microsoft/PowerToys/pull/9579

Threat `/` and `\` as root of system drive (typically `C:\`)
Threat `~` as user home directory `%USERPROFILE%`

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## PR Checklist

- [ ] Closes: #xxx
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
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signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
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repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
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## Detailed Description of the Pull Request / Additional comments

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<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

- Tested search starting with `~` `/` `\`
- Tested UNC network path starting with `\\...` and `//...`

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Shell/FallbackExecuteItem.cs

@@ -4,6 +4,7 @@
 
 using System;
 using System.IO;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CmdPal.Ext.Shell.Helpers;
@@ -15,6 +16,8 @@ namespace Microsoft.CmdPal.Ext.Shell;
 
 internal sealed partial class FallbackExecuteItem : FallbackCommandItem, IDisposable
 {
+    private static readonly char[] _systemDirectoryRoots = ['\\', '/'];
+
     private readonly Action<string>? _addToHistory;
     private CancellationTokenSource? _cancellationTokenSource;
     private Task? _currentUpdateTask;
@@ -80,8 +83,8 @@ internal sealed partial class FallbackExecuteItem : FallbackCommandItem, IDispos
         cancellationToken.ThrowIfCancellationRequested();
 
         var searchText = query.Trim();
-        var expanded = Environment.ExpandEnvironmentVariables(searchText);
-        searchText = expanded;
+        Expand(ref searchText);
+
         if (string.IsNullOrEmpty(searchText) || string.IsNullOrWhiteSpace(searchText))
         {
             Command = null;
@@ -184,8 +187,8 @@ internal sealed partial class FallbackExecuteItem : FallbackCommandItem, IDispos
     internal static bool SuppressFileFallbackIf(string query)
     {
         var searchText = query.Trim();
-        var expanded = Environment.ExpandEnvironmentVariables(searchText);
-        searchText = expanded;
+        Expand(ref searchText);
+
         if (string.IsNullOrEmpty(searchText) || string.IsNullOrWhiteSpace(searchText))
         {
             return false;
@@ -197,4 +200,57 @@ internal sealed partial class FallbackExecuteItem : FallbackCommandItem, IDispos
 
         return exeExists || pathIsDir;
     }
+
+    private static void Expand(ref string searchText)
+    {
+        if (searchText.Length == 0)
+        {
+            return;
+        }
+
+        var singleCharQuery = searchText.Length == 1;
+
+        searchText = Environment.ExpandEnvironmentVariables(searchText);
+
+        if (!TryExpandHome(ref searchText))
+        {
+            TryExpandRoot(ref searchText);
+        }
+    }
+
+    private static bool TryExpandHome(ref string searchText)
+    {
+        if (searchText[0] == '~')
+        {
+            var home = Environment.GetFolderPath(Environment.SpecialFolder.UserProfile);
+
+            if (searchText.Length == 1)
+            {
+                searchText = home;
+            }
+            else if (_systemDirectoryRoots.Contains(searchText[1]))
+            {
+                searchText = Path.Combine(home, searchText[2..]);
+            }
+
+            return true;
+        }
+
+        return false;
+    }
+
+    private static bool TryExpandRoot(ref string searchText)
+    {
+        if (_systemDirectoryRoots.Contains(searchText[0]) && (searchText.Length == 1 || !_systemDirectoryRoots.Contains(searchText[1])))
+        {
+            var root = Path.GetPathRoot(Environment.SystemDirectory);
+            if (root != null)
+            {
+                searchText = searchText.Length == 1 ? root : Path.Combine(root, searchText[1..]);
+                return true;
+            }
+        }
+
+        return false;
+    }
 }