NcSelect: selected option not announced to screen readers

[pringelmann]

Bug description

When using NcSelect with a screen reader:

• The currently selected option is not announced when the dropdown receives focus
• When navigating options inside the open dropdown, the screen reader does not
  re-announce the context (e.g. which question/label this dropdown belongs to)
• There is no indication of which option is currently selected when browsing
  the list

Steps to reproduce

1. Render an NcSelect with a label and a pre-selected option
2. Navigate to it using keyboard + screen reader (tested with Orca on Firefox)
3. Focus the select -> the selected value is not announced
4. Open the dropdown -> screen reader reads list items but gives no context and
   does not indicate the selected item

Expected behavior

• On focus: the selected value should be announced alongside the label
• In the open list: the currently selected option should be announced as selected

Additional info

• Reported upstream from: Accessibility issues nextcloud/forms#2858
• Tested with: Orca + Firefox