Electron platform - 'Missing Command Error' on API calls

# Bug Report

## Problem

### What is expected to happen?

File APIs working normally and return a valid Entry or a valid error code.

### What does actually happen?

When invoking any api, it will throw
```javascript
FileError {code: "Missing Command Error", message: undefined}
```
with
```javascript
Error: exec proxy not found for :: File :: $TheAPIYouInvoked
```

### Environment, Platform, Device
`cordova.platformId = 'electron'`
Linux x64, Ubuntu 18.04

## Checklist

- [x] I searched for existing GitHub issues
- [x] I updated all Cordova tooling to most recent version

# Suspected Cause
Tracing the error output, I suspect this is caused by inconsistencies in platform detection strategy?

Eg: In [src/browser/FileProxy.js](https://github.com/apache/cordova-plugin-file/tree/master/src/browser) and [www/resolveLocalFileSystemURI.js](https://github.com/apache/cordova-plugin-file/blob/master/www/resolveLocalFileSystemURI.js)
```javascript
//In src/browser/FileProxy.js

// For chrome we don't need to implement proxy methods
// All functionality can be accessed natively.
if (require('./isChrome')()) {
var pathsPrefix = {
// Read-only directory where the application is installed.
applicationDirectory: location.origin + '/', // eslint-disable-line no-undef
// Where to put app-specific data files.
dataDirectory: 'filesystem:file:///persistent/',
// Cached files that should survive app restarts.
// Apps should not rely on the OS to delete files in here.
cacheDirectory: 'filesystem:file:///temporary/'
};

exports.requestAllPaths = function (successCallback) {
successCallback(pathsPrefix);
};

require('cordova/exec/proxy').add('File', module.exports);
return;
}
```
```javascript
//In www/resolveLocalFileSystemURI.js

// For browser platform: not all browsers use overrided `resolveLocalFileSystemURL`.
function checkBrowser () {
if (cordova.platformId === 'browser' && require('./isChrome')()) { // eslint-disable-line no-undef
module.exports.resolveLocalFileSystemURL = window.resolveLocalFileSystemURL || window.webkitResolveLocalFileSystemURL;
return true;
}
return false;
}
if (checkBrowser()) {
return;
}
```
In src/browser/FileProxy.js, it assumes Electron have all the native APIs. So it choose to register nothing in the execProxy of Cordova. Meanwhile, other modules consider Electron not qualified (`cordova.platformId !== 'browser'`) and overrides its methods.

# Fixing Attempt
I've hacked src/browser/FileProxy.js to ensure it uses a consistent platform detection logic
```javascript
//In src/browser/FileProxy.js

// For chrome we don't need to implement proxy methods
// All functionality can be accessed natively.
if (cordova.platformId === 'browser' && require('./isChrome')()) { // FIX HERE

//..........

require('cordova/exec/proxy').add('File', module.exports);
return;
}
```
And at least the API call is OK. No more `Error: exec proxy not found for :: File :: `

Is there anyone who can confirm this issue? I'm not familiar with Cordova platform thus quite unsure about this fix.

Reproduce an Electron file API call and inspect src/browser/FileProxy.js alongside www/resolveLocalFileSystemURI.js, focusing on their platform checks and proxy registration. Compare the Electron and browser paths, then verify that the API returns an Entry or valid error code without an exec proxy-not-found error.

Written by the indexing model from the issue text.