Provide a way for Javascript code in a Webview to invoke Python functions

### What is the problem or limitation you are having?

It is possible for Python to invoke Javascript code in a web view; however, it is not possible for a Javascript function to call back to Python code.

### Describe the solution you'd like

PyQt5 WebView not work in iOS or Android so I hope toga can realize this function

this is pyqt5 solution

**index.html:**
```html

Webview Demo

function callPython() {
alert(api.print('print from JavaScript!'));
}

Call Python Code print

```

**webview.py:**
```python
import os

from PyQt5.QtCore import QUrl
from PyQt5.QtWebKit import QWebSettings
from PyQt5.QtWebKitWidgets import QWebView, QWebPage
from PyQt5.QtWidgets import QWidget

from posmesh.platform import API

class WebPage(QWebPage):
def __init__(self, parent=None):
super(WebPage, self).__init__(parent)
self.api = API()
self.mainFrame().javaScriptWindowObjectCleared.connect(self.populateJavaScriptWindowObject)
def populateJavaScriptWindowObject(self):
self.mainFrame().addToJavaScriptWindowObject('api', self.api)

class Webview(QWidget):
def __init__(self, url="", parent=None):
super(Webview, self).__init__(parent)
self.view = QWebView(self)
self.view.setPage(WebPage())
self.view.settings().setAttribute(QWebSettings.LocalContentCanAccessRemoteUrls, True)
self.view.load(QUrl.fromLocalFile(os.getcwd() + url))
self.view.show()

```

**api.py**
```python
from PyQt5.QtCore import pyqtSlot as Slot, QObject
class API(QObject):
@Slot(str, result=str)
def print(self, msg):
print(msg)
return 'printed'
```

### Describe alternatives you've considered

Haven't found a WebView jsbridge solution across iOS, Android, windows, and Linux yet using python.

### Additional context

_No response_

Start by reviewing the issue's index.html, webview.py, and api.py examples, then identify Toga's existing WebView APIs and platform implementations that would need to participate. Done would mean JavaScript in a WebView can invoke Python functions across the requested platforms, with the supported interface and behavior made explicit.

Written by the indexing model from the issue text.