Add support for extension `recommendations` and `unwantedRecommendations` in devcontainer.json `customizations:vscode:settings:extensions`

Currently extension `recommendations` and `unwantedRecommendations` can only be set in _.vscode/extensions.json_ file.

Adding support for setting those in _devcontainer.json_ under `customizations:vscode:settings:extensions` key would allow avoiding the need for creating (and adding to **git**) the _.vscode/extensions.json_ file.

Something like:
```json
{
"customizations": {
"vscode": {
"settings": {
"extensions": {
"recommendations": [],
"unwantedRecommendations": []
}
}
}
}
}
```

This would be the "equivalent" for `customizations:vscode:settings:launch` (that "mimics" _.vscode/launch.json_) and for `customizations:vscode:settings:tasks` (that "mimics" _.vscode/tasks.json_).

Beginnen Sie mit dem Vergleich der bestehenden Verarbeitung von customizations:vscode:settings:launch und customizations:vscode:settings:tasks mit .vscode/extensions.json und devcontainer.json. Fügen Sie Unterstützung für recommendations und unwantedRecommendations unter customizations:vscode:settings:extensions hinzu, wobei die Beispielstruktur als gleichwertige Konfiguration akzeptiert wird.

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