settings.json: Quotes are being removed from nested JSON object keys

🐛 bug report

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• I have read the Contributing Guidelines for this project.
• I agree to follow the Code of Conduct that this project
  adheres to.
• I have searched the issue tracker for an issue that matches the one I want
  to file, without success.

Description of the problem

When adding nested properties to settings.json, reloading the page after some amount of time or work removes double quotes from all property keys (see example).

I encountered this problem on my first day of usage. My only changes were to add a vim keybinding to settings.json, and nothing else, so hopefully the steps I've provided are reproducible.

Immediate reloads don't produce this error, so I'm not sure exactly what the trigger is, but after every period I've actually coded for a bit, the error has occurred.

To Reproduce

1. Open a sandbox and enable vim
2. Add the following to settings.json:

  "vim.insertModeKeyBindings": [
    {
      "before": [
        "j",
        "k",
      ],
      "after": [
        "<esc>",
      ],
    },
    {
      "before": [
        "j",
        "j",
      ],
      "after": [
        "<esc>",
      ],
    },
  ],

3. Do some work (?).
4. Reload the page.
5. Error popup: Configuration: undefined missing 'after' key or 'command'.
6. Settings.json now looks like this:

"vim.insertModeKeyBindings": [
    {
      before: [
        "j",
        "k",
      ],
      after: [
        "<esc>",
      ],
    },
    {
      before: [
        "j",
        "j",
      ],
      after: [
        "<esc>",
      ],
    },
  ],

Your Environment

| Software | Name/Version | MacOS Big Sur 11.3
| ---------------- | ------------ |
| Сodesandbox | |
| Browser | Firefox |
| Operating System | MacBook Pro 2019 16-inch |

Start by reproducing the issue with the nested settings.json vim.insertModeKeyBindings example and the reload sequence described. Trace how settings.json is saved and reloaded, then verify that nested property keys retain their double quotes and the configuration error no longer appears.