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/trailing_slash.rs

@@ -0,0 +1,108 @@
+use std::{
+    borrow::Cow,
+    path::{Path, PathBuf},
+};
+
+/// Strip any trailing slashes from a non-root path. This is required in some contexts, for example
+/// for the `PWD` environment variable.
+pub fn strip_trailing_slash(path: &Path) -> Cow<Path> {
+    if has_trailing_slash(path) {
+        // If there are, the safest thing to do is have Rust parse the path for us and build it
+        // again. This will correctly handle a root directory, but it won't add the trailing slash.
+        let mut out = PathBuf::with_capacity(path.as_os_str().len());
+        out.extend(path.components());
+        Cow::Owned(out)
+    } else {
+        // The path is safe and doesn't contain any trailing slashes.
+        Cow::Borrowed(path)
+    }
+}
+
+/// `true` if the path has a trailing slash, including if it's the root directory.
+#[cfg(windows)]
+pub fn has_trailing_slash(path: &Path) -> bool {
+    use std::os::windows::ffi::OsStrExt;
+
+    let last = path.as_os_str().encode_wide().last();
+    last == Some(b'\\' as u16) || last == Some(b'/' as u16)
+}
+
+/// `true` if the path has a trailing slash, including if it's the root directory.
+#[cfg(unix)]
+pub fn has_trailing_slash(path: &Path) -> bool {
+    use std::os::unix::ffi::OsStrExt;
+
+    let last = path.as_os_str().as_bytes().last();
+    last == Some(&b'/')
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[cfg_attr(not(unix), ignore = "only for Unix")]
+    #[test]
+    fn strip_root_unix() {
+        assert_eq!(Path::new("/"), strip_trailing_slash(Path::new("/")));
+    }
+
+    #[cfg_attr(not(unix), ignore = "only for Unix")]
+    #[test]
+    fn strip_non_trailing_unix() {
+        assert_eq!(
+            Path::new("/foo/bar"),
+            strip_trailing_slash(Path::new("/foo/bar"))
+        );
+    }
+
+    #[cfg_attr(not(unix), ignore = "only for Unix")]
+    #[test]
+    fn strip_trailing_unix() {
+        assert_eq!(
+            Path::new("/foo/bar"),
+            strip_trailing_slash(Path::new("/foo/bar/"))
+        );
+    }
+
+    #[cfg_attr(not(windows), ignore = "only for Windows")]
+    #[test]
+    fn strip_root_windows() {
+        assert_eq!(Path::new(r"C:\"), strip_trailing_slash(Path::new(r"C:\")));
+    }
+
+    #[cfg_attr(not(windows), ignore = "only for Windows")]
+    #[test]
+    fn strip_non_trailing_windows() {
+        assert_eq!(
+            Path::new(r"C:\foo\bar"),
+            strip_trailing_slash(Path::new(r"C:\foo\bar"))
+        );
+    }
+
+    #[cfg_attr(not(windows), ignore = "only for Windows")]
+    #[test]
+    fn strip_non_trailing_windows_unc() {
+        assert_eq!(
+            Path::new(r"\\foo\bar"),
+            strip_trailing_slash(Path::new(r"\\foo\bar"))
+        );
+    }
+
+    #[cfg_attr(not(windows), ignore = "only for Windows")]
+    #[test]
+    fn strip_trailing_windows() {
+        assert_eq!(
+            Path::new(r"C:\foo\bar"),
+            strip_trailing_slash(Path::new(r"C:\foo\bar\"))
+        );
+    }
+
+    #[cfg_attr(not(windows), ignore = "only for Windows")]
+    #[test]
+    fn strip_trailing_windows_unc() {
+        assert_eq!(
+            Path::new(r"\\foo\bar"),
+            strip_trailing_slash(Path::new(r"\\foo\bar\"))
+        );
+    }
+}