This PR implements PWD-per-drive as described in discussion #14355

Description

On Windows, CMD or PowerShell assigns each drive its own current directory. For example, if you are in 'C:\Windows', switch to 'D:', and navigate to 'D:\Game', you can return to 'C:\Windows' by simply typing 'C:'.

This PR enables Nushell on Windows to have the same capability, allowing each drive to maintain its own PWD (Present Working Directory).

User-Facing Changes

Currently, 'cd' or 'ls' only accept absolute paths if the path starts with 'C:' or another drive letter. With PWD-per-drive, users can use 'cd' (or auto cd) and 'ls' in the same way as 'cd' and 'dir' in PowerShell, or similarly to 'cd' and 'dir' in CMD (noting that cd in CMD has slightly different behavior, 'cd' for another drive only changes current directory of that drive, but does not switch there).

Interaction example on switching between drives:

~>D:
D:\>cd Test
D:\Test\>C:
~>D:
D:\Test\>C:
~>cd D:..
D:\>C:x/../y/../z/..
~>cd D:Test\Test
D:\Test\Test>C:
~>D:...
D:\>

Interaction example on auto-completion at cmd line:

~>cd D:\test[Enter]
D:\test>~[Enter]
~>D:[TAB]
~>D:\test[Enter]
D:\test>c:.c[TAB]
c:\users\nushell\.cargo\ c:\users\nushell\.config\

Interaction example on pass PWD-per-drive to child process: (Note CMD will use it, but PowerShell will ignore it though it still prepares such info for child process)

~>cd D:\Test
D:\Test>cd E:\Test
E:\Test\>~
~>CMD
Microsoft Windows [Version 10.0.22631.4460]
(c) Microsoft Corporation. All rights reserved.

C:\Users\Nushell>d:
D:\Test>e:
E:\Test>

Brief Change Description

1.Added 'crates/nu-path/src/pwd_per_drive.rs' to implement a 26-slot array mapping drive letters to PWDs. Test cases are included in the same file, along with a doctest for the usage of PWD-per-drive.
2. Modified 'crates/nu-path/src/lib.rs' to declare module of pwd_per_drive and export struct for PWD-per-drive.
3. Modified 'crates/nu-protocol/src/engine/stack.rs' to sync PWD when set_cwd() is called. Add PWD-per-drive map as member. Clone between parent and child. Stub/proxy for nu_path::expand_path_with() to facilitate filesystem commands using PWD-per-drive.
4. Modified 'crates/nu-cli/src/repl.rs' auto_cd uses PWD-per-drive to expand path.
5. Modified 'crates/nu-cli/src/completions/completion_common.rs' to expand relative path when press [TAB] at command line.
6. Modified 'crates/nu-engine/src/env.rs' to collect PWD-per-drive info as env vars for child process as CMD or PowerShell do, this can let child process inherit PWD-per-drive info.
7. Modified 'crates/nu-engine/src/eval.rs', caller clone callee's PWD-per-drive info, supporting 'def --env'
8. Modified 'crates/nu-engine/src/eval_ir.rs', 'def --env' support. Remove duplicated fn redirect_env()
9. Modified 'src/run.rs', to init PWD-per-drive when startup.

filesystem commands that modified:

1. Modified 'crates/nu-command/src/filesystem/cd.rs', 1 line change to use stackscoped PWD-per-drive.
   Other commands, commit pending....

Local test def --env OK:

E:\study\nushell> def --env env_cd_demo [] {                 
:::     cd ~
:::     cd D:\Project
:::     cd E:Crates
::: }
E:\study\nushell>                                                   
E:\study\nushell> def cd_no_demo [] {                   
:::     cd ~
:::     cd D:\Project
:::     cd E:Crates
::: }
E:\study\nushell> cd_no_demo                                 
E:\study\nushell> C:
C:\>D:
D:\>E:                                     
E:\study\nushell>env_cd_demo
E:\study\nushell\crates> C:
~>D:
D:\Project>E:                                     
E:\study\nushell\crates>     

Tests + Formatting

• cargo fmt --all -- --check passed.
• cargo clippy --workspace -- -D warnings -D clippy::unwrap_used passed.
• cargo test --workspace passed on Windows developer mode and Ubuntu.
• cargo run -- -c "use toolkit.nu; toolkit test stdlib" passed.
• nushell:

> use toolkit.nu  # or use an `env_change` hook to activate it automatically
> toolkit check pr
> ```
passed