Navigation dropdowns do not report expanded/collapsed state

**Describe the bug**

Navigation dropdowns do not report expanded/collapsed state.

**To Reproduce**

Steps to reproduce the behavior:

1. Go to https://vanillaframework.io/docs/examples/patterns/navigation/dropdown-dark?theme=light
2. Observe that the nested `p-navigation__item--dropdown-toggle` elements do not have `aria-expanded` attribution on them.
3. Install a screen reader
4. Access the above navigation bar using keyboard controls, and tab over the navigation dropdowns.
5. Verify that the screen reader does not report expanded/collapsed state.

**Expected behavior**

The navigation dropdown links report expanded/collapsed state to screen readers. Tabbing over them should report "button collapsed" or something similar. Expanding/collapsing them should also report the change to the screen reader.

See https://getbootstrap.com/docs/4.0/components/navbar/ for example.

**Desktop (please complete the following information):**

- OS: PopOS 22.04
- Browser Chrome
- Version 125.0.6422.141 x64

**Additional context**

This was noticed during [work on reduced top-nav](https://github.com/canonical/vanilla-framework/pull/5107#discussion_r1661262171), but effects all vanilla navigation.

Start with the navigation dropdown example at the linked Vanilla Framework URL and inspect the nested p-navigation__item--dropdown-toggle elements, including the reduced top-nav work referenced in pull request 5107. Verify keyboard and screen-reader behavior, then ensure the dropdown links report their expanded or collapsed state and announce changes when toggled.

Written by the indexing model from the issue text.