Expand multiple dots in path in completions (#13725)

# Description
This is my first PR, and I'm looking for feedback to help me improve! 

This PR fixes #13380 by expanding the path prior to parsing it.
Also I've removed some unused code in
[completion_common.rs](84e92bb02c/crates/nu-cli/src/completions/completion_common.rs
)
# User-Facing Changes

Auto-completion for "cd .../" now works by expanding to "cd ../../". 

# Tests + Formatting

Formatted and added 2 tests for triple dots in the middle of a path and
at the end.
Also added a test for the expand_ndots() function.

crates/nu-path/src/dots.rs

@@ -152,6 +152,17 @@ mod test_expand_ndots {
         };
         assert_path_eq!(expand_ndots(path), expected);
     }
+
+    #[test]
+    fn trailing_dots() {
+        let path = Path::new("/foo/bar/..");
+        let expected = if cfg!(windows) {
+            r"\foo\bar\.."
+        } else {
+            "/foo/bar/.."
+        };
+        assert_path_eq!(expand_ndots(path), expected);
+    }
 }
 
 #[cfg(test)]