feat: cmd.exe detection heuristic

[samypr100]

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"

[charliermarsh]

Maybe we should just merge shlex_posix and shlex_windows, and pass in Shell to toggle the behavior -- wdyt?

[samypr100]

I kinda like the separation between Posix and Windows, but I'm biased 😆

[charliermarsh]

Sounds good :)

[charliermarsh]

Thanks!