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-command/src/filesystem/cd.rs

@@ -100,6 +100,9 @@ impl Command for Cd {
             }
         };
 
+        // Strip the trailing slash from the new path. This is required for PWD.
+        let path = nu_path::strip_trailing_slash(&path);
+
         // Set OLDPWD.
         // We're using `Stack::get_env_var()` instead of `EngineState::cwd()` to avoid a conversion roundtrip.
         if let Some(oldpwd) = stack.get_env_var(engine_state, "PWD") {