Catch xhr request with new js files

Hi,

Currently we're using twig for get some js files for the views. But the debugger isn't catching the new files, and the breakpoints are always in gray.

Firefox dev version: 82.0b9 (64-bit)
Vscode version: 1.50.0

twig request code:
```

{% if javascript|length %}
{% for script in javascript %}

{% endfor %}
{% endif %}

```

launch.json
```json
{
"name": "Listen for Firefox",
"type": "firefox",
"request": "launch",
"reAttach": true,
"profile": "dev-edition-default",
"url": "http://localhost/",
"pathMappings": [
{
"url": "http://localhost/",
"path": "${workspaceFolder}/"
}
],
"skipFiles": [
"${workspaceFolder}/**/*.min.js"
],
"log": {
"fileName": "${workspaceFolder}/logs/firefox-debug-log.txt",
"fileLevel": {
"default": "Debug"
}
}
}
```

Reproduce the issue with the Twig-generated script tags and the launch.json configuration, especially its pathMappings, using Firefox Dev Edition and VS Code. Trace how newly requested JavaScript files are discovered and mapped by the debugger; done means breakpoints in those files bind instead of remaining gray.

Scritto dal modello di indicizzazione a partire dal testo della issue.