Checkbox quick pick hover doesn't support icons

[Tyriar]

[TylerLeonhardt]

Verification:

• Create an extension with this:

  await window.showQuickPick(['$(gear) one'], {
  	canPickMany: true,
  	placeHolder: 'hover over the checkbox',
  });

• show it and hover over the checkbox
• it should not have the gear in the hover (this is because the hover currently just passes in a string, I may change this to pass in a markdown string so that the gear can be rendered, but without modifying core at all this would verify this bug)

[meganrogge]