Allows cd (and other commands that depend on current working directory) to use path of type '~user' (#5323)

* Added search terms to math commands

* Attempts to add ~user.

From: // Extend this to work with "~user" style of home paths

* Clippy recommendation

* clippy suggestions, again.

* fixing non-compilation on windows and macos

* fmt apparently does not like my imports

* even more clippy issues.

* less expect(), single conversion, match. Should work for MacOS too.

* Attempted to add functionality for windows: all it does is take the home path of current user, and replace the username.

* silly mistake in Windows version of user_home_dir()

* Update tilde.rs

* user_home_dir now returns a path instead of a string - should be smoother with no conversions to string

* clippy warnings

* clippy warnings 2

* Changed user_home_dir to return PathBuf now.

* Changed user_home_dir to return PathBuf now.

* forgot to fmt

* fixed windows build errors from modifying pathbuf but not returning it

* fixed windows clippy errors from returning () instead of pathbuf

* forgot to fmt

* borrowed path did not live long enough.

* previously, path.push did not work because rest_of_path started with "/" - it was not relative. Removing the / makes it a relative path again.

* Issue fixed.

* Update tilde.rs

* fmt.

* There is now a zero chance of panic. All expect()s have been removed.

* Patched join_path_relative to accommodate ~user paths. Previously, /some/path/~user might have been passed on; now, ~user is taken as absolute.

* fmt

* clippy errors

crates/nu-path/src/expansions.rs

@@ -18,8 +18,8 @@ where
         // more ugly - so we don't do anything, which should result in an equal
         // path on all supported systems.
         relative_to.into()
-    } else if path.starts_with("~") {
-        // do not end up with "/some/path/~"
+    } else if path.to_string_lossy().as_ref().starts_with('~') {
+        // do not end up with "/some/path/~" or "/some/path/~user"
         path.into()
     } else {
         relative_to.join(path)