Add PWD to the `Reedline` state

[yizhepku]

This PR adds the current working directory as part of the Reedline state, instead of using std::env::current_dir() to retrieve it from the environment. Applications should call Reedline::with_cwd() to update the current working directory when it changes.

This is necessary to support https://github.com/nushell/nushell/pull/12922, which causes std::env::current_dir() to deviate from Nushell's $env.PWD.