feat: cmd.exe detection heuristic (#2226)
## Summary
Follow up from discussion in https://github.com/astral-sh/uv/pull/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"
```
