Add documentation on how to use this plugin with TypeScript

# Feature Request

## Motivation Behind Feature

I'm migrating an exiting project to Cordova, and this plugin has been challenging. The project uses TypeScript and figuring out how to use the types in the package properly has been a hassle.

(I don't use 'global' libraries very often. My current solution is simply `import 'cordova-plugin-file';` It does fail at constants like `cordova.file.dataDirectory`.)

## Feature Description

A paragraph in the documentation about using this plugin with TypeScript and a complete TypeScript module (file) using it as an example would be nice.

## Alternatives or Workarounds

none

Review the plugin's existing documentation and the reported TypeScript import and constant usage. Done means adding a paragraph explaining TypeScript usage and a complete TypeScript module example that addresses the documented usage problem.

Written by the indexing model from the issue text.