Wrap `mapdl.directory` to check for files

So it will be awesome that we could do:

```py
if (mapdl.directory / "file.rst").is_file():
# File.rst exist
```

We would have to subclass `PurePath` class.

Start by locating the implementation of mapdl.directory and reviewing how it currently represents the directory. Read Python's PurePath behavior, then determine the scope needed for path joining and is_file() checks; the work is done when the example in the issue works and relevant behavior is covered by tests.

Written by the indexing model from the issue text.