Promises

# Feature Request
I want to add Promise functionality to this plugin and have some questions.

From my point, Promises are save to add natively, because:
- For Android the minimum supported Android version is 7.0 since cordova-android 12. Promises are supported since [Version 4.4.4](https://caniuse.com/?search=Promise)
- For iOS, WKWebView is the required minimum. Promises are supported since [iOS 8](https://caniuse.com/?search=Promise).

Here a sample of a conversion i would make.

Old code:
```javascript
FileEntry.prototype.file = function (successCallback, errorCallback) {
const localURL = this.toInternalURL();
const win = successCallback && function (f) {
const file = new File(f.name, localURL, f.type, f.lastModifiedDate, f.size);
successCallback(file);
};
const fail = errorCallback && function (code) {
errorCallback(new FileError(code));
};
exec(win, fail, 'File', 'getFileMetadata', [localURL]);
};
```

New code:
```javascript
FileEntry.prototype.file = function (successCallback, errorCallback) {
return new Promise((resolve, reject) => {
const localURL = this.toInternalURL();
const win = function (f) {
const file = new File(f.name, localURL, f.type, f.lastModifiedDate, f.size);
if (successCallback) successCallback(file);
resolve(file);
};
const fail = function (code) {
if (errorCallback) errorCallback(new FileError(code));
reject(new FileError(code));
};
exec(win, fail, 'File', 'getFileMetadata', [localURL]);
});
};
```
Would it be fine like this?

Regards

The concrete entry point shown is FileEntry.prototype.file in the issue sample; begin by reviewing the plugin's existing callback-based APIs and the stated Android and iOS compatibility constraints. The Promise API scope and expected behavior still need agreement in the discussion, and done would require an agreed implementation plan plus validation for the affected APIs.

Written by the indexing model from the issue text.