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

[rubensa]

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:

{
  "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).