getParent returns always root directory

# Bug Report

## Problem

`getParent()` returns always the root directory instead of the parent directory.

### What is expected to happen?
`getParent()` on directory `/documents/upload/foobar/` should return `/documents/upload/`

### What does actually happen?
`getParent()` return `/`

## Information

### Command or Code
https://github.com/apache/cordova-plugin-file/blob/master/src/windows/FileProxy.js#L553

```js
(new RegExp('/[^/]*/?$')).test(path)
```
is always true

### Environment, Platform, Device
UWP Windows 10 18362.449

### Version information
cordova: 8.1.2
cordova-windows: 6.0.1
cordova-plugin-file: 5.00

## Checklist

- [x] I searched for existing GitHub issues
- [x] I updated all Cordova tooling to most recent version
- [x] I included all the necessary information above

Start in src/windows/FileProxy.js around line 553 and inspect the getParent() path-matching logic. Reproduce the documented case with /documents/upload/foobar/ and verify that the result is /documents/upload/ rather than /. Done means the Windows implementation returns the immediate parent directory for nested paths.

Written by the indexing model from the issue text.