settings: break down indexing options

[radeksimko]

As per our internal conversation yesterday, this allows us to get best of both worlds where VS Code can present nice settings UI and LS isn't limited to a flat structure.

I will raise a separate PR to update all three terraform* options to follow the same principles as documented and further align it with https://github.com/hashicorp/vscode-terraform/blob/main/docs/settings-migration.md , i.e.

instead of

{
  "terraformExecPath": "",
  "terraformExecTimeout": "",
  "terraformExecLogFilePath": ""
}

we will accept

{
  "terraform": {
    "path": ""
    "timeout": ""
    "logFilePath": ""
  }
}

and any future (stable, non-experimental) settings can follow these principles.

[dbanck]

LGTM 👍 I like the approach of handling settings this way

[github-actions]

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