* Avoid extending the directory without globs in `nu_engine::glob_from`
* avoid joining a `*` to the directory without globs
* remove checks on directory permission and whether it is empty
The previous implemention of `nu_engine::glob_from` will extend the
given directory even if it containes no glob pattern. This commit
overcomes lack of consistency with the function `nu_glob::glob`.
* Add flag -D to ls, to list the directory itself instead of its contents
* add --directory (-d) flag to ls
* correct the difference between the given path and the cwd
* set default path to `.` instead of `./*` when --directory (-d) flag is true
* add comments
* add an example
* add tests
* fmt
