vscode: Setting sync: Sign in dialogs not accessibility friendly

From https://github.com/microsoft/vscode/issues/120012 by @neurrone

1. New vscode-insiders with fresh user data dir
2. Turn on settings sync, you get the Please Sign In quick pick dialog where you can check off which items you want to synchronize

This dialog is very unnacessible. Problems:

• Once the dialog is created the “Please sign in to synchronize” part is not being read out by screen reader. I recommend to use a placeholder since that would nicely be read.
• Down does not move focus to the Checkboxes
• It is hard for the user to figure out there are checkboxes below. Aria label of the input box should contain this information
• Checkbox list items do not have aria-checked set on them. For example on how to do this, checkout breakpoints

fyi @chrmarti @TylerLeonhardt