pyinputplus.inputFilepath(path, mustExist=True) does not validate that a filepath exists on the system.

Wanted to use pyinputplus.inputFilepath in an exercise from ATBS, however, it does not actually validate that a path exists, it simply
returns the path as a string, nor does it seem to validate an actual filepath, it simply returns whatever value is entered as a string. This does not seem to be intended behavior.

```
>>> import pyinputplus as pyip
>>> pyip.inputFilepath("Enter a valid directory: ", mustExist=True)
Enter a valid directory: /foo/bar/bang/
'/foo/bar/bang/'
>>> pyip.inputFilepath("Enter a valid directory: ", mustExist=True)
Enter a valid directory: Test directory does not exist
'Test directory does not exist'
>>>
```

Start by locating and reading the inputFilepath implementation, then reproduce the two examples from the issue with mustExist=True. Done means nonexistent paths and non-filepath text are rejected instead of returned unchanged, while valid input continues to work.

Written by the indexing model from the issue text.