Allow globally defined snippets in overrides.json

[m4dh4t]

Currently if we want to provide JupyterLab's users globally defined snippets by setting them inside <sys.prefix>/local/share/jupyter/lab/settings/overrides.json (see JupyterLab's documentation), these are ignored until manually set inside ~/.jupyter/lab/user-settings/jupyterlab-code-snippets/snippets.jupyterlab-settings via the widget or the settings GUI.

The goal of this commit is to allow snippets initialization through extension overridden settings without user interaction at Lab startup.

Dummy overrides.json example:

{
    "jupyterlab-code-snippets:snippets": {
        "snippets": [
            {
                "id": 1,
                "tags": [],
                "name": "Hello World",
                "language": "Python",
                "code": "print('Hello, World!')"
            }
        ]
    }
}