Fix trailing slash in PWD set by cd (#12760)

# Description

Fixes #12758.

#12662 introduced a bug where calling `cd` with a path with a trailing
slash would cause `PWD` to be set to a path including a trailing slash,
which is not allowed. This adds a helper to `nu_path` to remove this,
and uses it in the `cd` command to clean it up before setting `PWD`.

# Tests + Formatting
I added some tests to make sure we don't regress on this in the future.

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

crates/nu-path/src/lib.rs

@@ -4,8 +4,10 @@ pub mod dots;
 pub mod expansions;
 mod helpers;
 mod tilde;
+mod trailing_slash;
 
 pub use components::components;
 pub use expansions::{canonicalize_with, expand_path_with, expand_to_real_path, locate_in_dirs};
 pub use helpers::{config_dir, config_dir_old, home_dir};
 pub use tilde::expand_tilde;
+pub use trailing_slash::{has_trailing_slash, strip_trailing_slash};