Fix navbar keyboard accessibility

[sarayourfriend]

Fixes

Fixes #116 by @sarayourfriend

Description

Adds a Dropdown component based on Vuetensils' Dropdown to make the navbar keyboard accessible.

Technical details

Heavily modifies the Vuetensils' Dropdown to accomodate a couple of things:

0. Make the keyboard navigation behave the same as WordPress.org's navbar (i.e., no arrow key navigation, just tabbing through each item in order)
1. Adds focus management to each child by making an onFocus prop available through the default slot. This onFocus prop should be used in the @focus for each dropdown item. This makes it possible to tab "backwards" from a subsequent navbar item. For example, in the real example of the Openverse dropdown if you Shift + Tab from the Web Extension navbar item, you should focus into the last item of the Resources dropdown, not the Resources dropdown itself.
2. Makes the dropdown container always rendered into the DOM by using the visually hidden styles, lifted from @wordpress/components. This makes the dropdown items always appear in the DOM, making them keyboard focusable while hiding them from sighted users.
3. Use Bulma classes to fit the existing implementation. This is a drop-in replacement for the existing implementation, style wise, but adds keyboard accessibility (there should be no visual differences between this and the existing implementation).

Tests

If you're on macOS, ensure that you've enabled keyboard navigation in the accessibility options following the instructions here: https://www.a11yproject.com/posts/2017-12-29-macos-browser-keyboard-navigation/

After restarting your browser, you should be able to tab through the navbar items including the dropdown items. Compare this to the live site and you'll see that it was not previously possible.

There are also unit tests to ensure that the focus management works (by teseting the presence of the visible class on the dropdown wrapper).

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main or master).
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• [N/A] I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

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[obulat]

This is fantastic, Sara. I have left some tiny comments and questions inline. So great to improve a11y on such a vital part of the site!

[zackkrida]

LGTM! I appreciate the jest --watch script as well.