Handle top-level preference access from plugins

[colin-grant-work]

What it does

Fixes #12043

How to test

Follow the steps in #12043, but observe that there are no keys that include the string "undefined".

You can use this plugin and the command 'Access preferences at the top level'

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

LGTM 👍

2023-01-10T17:32:29.378Z root INFO [hosted-plugin: 2253741] {
  key: 'editor.tabSize',
  defaultValue: 4,
  globalValue: 8,
  workspaceValue: undefined,
  workspaceFolderValue: undefined
}
2023-01-10T17:32:29.437Z root INFO [hosted-plugin: 2253741] {
  key: 'tabSize',
  defaultValue: 4,
  globalValue: 8,
  workspaceValue: undefined,
  workspaceFolderValue: undefined
}