[wip] implement accessible dropdown selection

[andrewharvey]

Working on making the dropdown selection more accessible for screen readers. I need to do more testing and fix some minor issues.

[tristen]

@andrewharvey is this ready to go? code lgtm

[andrewharvey]

@tristen Not yet, I'm still working to resolve:

• The "Example with options" demo, ChromeVox reads the suggestions as "Object object".
• The "Simple example" demo, when moving through the list with the up and down keys, it moves the cursor in the input, which causes ChromeVox to sometimes read out the text you've entered so far and then the list item when moving through them.

However all my other PR's for suggestions and for mapbox-gl-geocoder are ready for review.

[andrewharvey]

It looks like there's a new ARIA standard for implementing accessibility for these drop down lists, with the approach implementing in this PR now legacy.

https://www.w3.org/TR/wai-aria-practices/examples/listbox/listbox-collapsible.html
https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/listbox_role

It looks like the new standard requires using id's for these elements which might create problems when more than one suggestions is in use on the same page. So unfortunately I'm going to shelf this PR for now.