feat: cmd.exe detection heuristic (#2226)

## Summary

Follow up from discussion in #2223

Detect CMD.exe by checking if `PROMPT` env var is set on windows,
otherwise assume it's PowerShell.

Note, this will not work if user modifies their system env vars to
include `PROMPT` by default or if they launch nested PowerShell from
Command Prompt (e.g. `Developer PowerShell for VS 2022`).

## Test Plan

Only tested locally, although we try to add some CI tests that
specifically use CMD.exe

Command Prompt
```
Microsoft Windows [Version 10.0.19044.3086]
(c) Microsoft Corporation. All rights reserved.

Z:\Users\samypr100\dev\uv>Z:\Users\samypr100\.cargo\bin\cargo.exe +stable run --color=always -- venv "Foo Bar"
    Finished dev [unoptimized + debuginfo] target(s) in 0.69s
     Running `target\debug\uv.exe venv "Foo Bar"`
Using Python 3.12.2 interpreter at: Z:\Users\samypr100\AppData\Local\Programs\Python\Python312\python.exe
Creating virtualenv at: Foo Bar
Activate with: "Foo Bar\Scripts\activate"
```

Power Shell
```
Windows PowerShell
Copyright (C) Microsoft Corporation. All rights reserved.

Try the new cross-platform PowerShell https://aka.ms/pscore6

PS Z:\Users\samypr100\dev\uv>Z:\Users\samypr100\.cargo\bin\cargo.exe +stable run --color=always -- venv "Foo Bar"
    Finished dev [unoptimized + debuginfo] target(s) in 0.63s
     Running `target\debug\uv.exe venv "Foo Bar"`
Using Python 3.12.2 interpreter at: Z:\Users\samypr100\AppData\Local\Programs\Python\Python312\python.exe
Creating virtualenv at: Foo Bar
Activate with: & "Foo Bar\Scripts\activate"
```

crates/uv/src/commands/venv.rs

@@ -230,7 +230,14 @@ async fn venv_impl(
             "source {}",
             shlex_posix(path.join("bin").join("activate.csh"))
         )),
-        Some(Shell::Powershell) => Some(shlex_windows(path.join("Scripts").join("activate"))),
+        Some(Shell::Powershell) => Some(shlex_windows(
+            path.join("Scripts").join("activate"),
+            Shell::Powershell,
+        )),
+        Some(Shell::Cmd) => Some(shlex_windows(
+            path.join("Scripts").join("activate"),
+            Shell::Cmd,
+        )),
     };
     if let Some(act) = activation {
         writeln!(printer.stderr(), "Activate with: {}", act.green()).into_diagnostic()?;
@@ -254,15 +261,20 @@ fn shlex_posix(executable: impl AsRef<Path>) -> String {
     }
 }
 
-/// Quote a path, if necessary, for safe use in `PowerShell`.
-fn shlex_windows(executable: impl AsRef<Path>) -> String {
+/// Quote a path, if necessary, for safe use in `PowerShell` and `cmd`.
+fn shlex_windows(executable: impl AsRef<Path>, shell: Shell) -> String {
     // Convert to a display path.
     let executable = executable.as_ref().simplified_display().to_string();
 
-    // Wrap the executable in quotes (and a `&` invocation) if it contains spaces.
-    // TODO(charlie): This won't work in `cmd.exe`.
+    // Wrap the executable in quotes (and a `&` invocation on PowerShell), if it contains spaces.
     if executable.contains(' ') {
-        format!("& \"{executable}\"")
+        if shell == Shell::Powershell {
+            // For PowerShell, wrap in a `&` invocation.
+            format!("& \"{executable}\"")
+        } else {
+            // Otherwise, assume `cmd`, which doesn't need the `&`.
+            format!("\"{executable}\"")
+        }
     } else {
         executable
     }

crates/uv/src/shell.rs

@@ -9,6 +9,8 @@ pub(crate) enum Shell {
     Fish,
     /// PowerShell
     Powershell,
+    /// Cmd (Command Prompt)
+    Cmd,
     /// Z SHell (zsh)
     Zsh,
     /// Nushell
@@ -34,7 +36,14 @@ impl Shell {
         } else if let Some(env_shell) = std::env::var_os("SHELL") {
             Shell::from_shell_path(env_shell)
         } else if cfg!(windows) {
-            Some(Shell::Powershell)
+            // Command Prompt relies on PROMPT for its appearance whereas PowerShell does not.
+            // See: https://stackoverflow.com/a/66415037.
+            if std::env::var_os("PROMPT").is_some() {
+                Some(Shell::Cmd)
+            } else {
+                // Fallback to PowerShell if the PROMPT environment variable is not set.
+                Some(Shell::Powershell)
+            }
         } else {
             None
         }