Adjust type definitions to work without type imports

# Bug Report

## Problem

### What is expected to happen?

The following code should compile without adding any imports:

```
export async function resolveLocalFileSystemURL(url: string): Promise {
if ('function' !== typeof window?.resolveLocalFileSystemURL) {
throw new Error('window.resolveLocalFileSystemURL is not a function');
}
return new Promise((resolve, reject) => window.resolveLocalFileSystemURL(url, resolve, reject));
}

export async function getDirectory(directoryEntry: DirectoryEntry,
path: string, options?: Flags): Promise {
return new Promise((resolve, reject) => directoryEntry.getDirectory(path, options, resolve, reject));
}
```

### What does actually happen?

```
error TS2304: Cannot find name 'Entry'.
error TS2339: Property 'resolveLocalFileSystemURL' does not exist on type 'Window & typeof globalThis'.
error TS2339: Property 'resolveLocalFileSystemURL' does not exist on type 'Window & typeof globalThis'.
error TS2304: Cannot find name 'DirectoryEntry'.
error TS2304: Cannot find name 'Flags'.
error TS2304: Cannot find name 'DirectoryEntry'.
```

## Information

It seems that I cannot use this plugin without explicitly importing the type declarations. But (most) import syntaxes are meant to import types **and implementations** – the latter not being provided in any way by npm standards.

(When I add `import 'cordova-plugin-file';` it tells me that the definition file is no module. `/// ` works, but arriving at this workaround took quite a while.)

When I use _cordova-sqlite-storage_ with the types from _DefinitelyTyped_, it works out of the box and without any imports:

```
export async function openDatabase(args: SQLitePlugin.OpenArgs): Promise {
if ('function' !== typeof window?.sqlitePlugin?.openDatabase) {
throw new Error('window.sqlitePlugin.openDatabase is not a function');
}
return new Promise((resolve, reject) =>
window.sqlitePlugin.openDatabase(args, db => resolve(new AsyncDatabase(db)), reject));
}
```
The difference is likely the namespace declaration in _cordova-sqlite-storage's_ type definition file.

### Version information

* cordova-plugin-file 6.0.2
* typescript 3.8.3

## Checklist

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

Inspect the plugin's type declaration and compare it with the cordova-sqlite-storage DefinitelyTyped declaration, especially its namespace pattern. Reproduce the TypeScript 3.8.3 errors using the example; done when the shown code compiles without explicit type imports and the declarations expose the Cordova file types.

Written by the indexing model from the issue text.