Disable WebView debug modes by default and expose configuration to enable it

### Describe the bug

Currently, a "debug mode" is enabled for the WebView on each platform. For Windows, macOS, iOS, and Gtk, this is explicitly enabled by Toga; for Android, this debug mode should be controlled by whether the running build is debug or release. Ideally, the WebView would not be configured this way in distributed apps using Toga....but developers using Toga do not have a way to disable it.

### Steps to reproduce

Start an app using the WebView and access its "dev tools" at runtime.

### Expected behavior

Debugging tools/settings for WebView are disabled for released apps. However, developers should have the option to enable these tools during their development.

### Screenshots

_No response_

### Environment

- Operating System: all
- Python version: any
- Software versions:
- Toga: `0.4.6.dev0+g9a8d7a903`

### Logs

_No response_

### Additional context

_No response_

Start by locating Toga's WebView setup for Windows, macOS, iOS, Gtk, and Android, including how platform build mode is detected. Trace the dev-tools entry point and determine how configuration could keep released apps disabled while allowing development builds to opt in; done means behavior is consistent across all listed platforms.

Written by the indexing model from the issue text.