Screenreader users can't type text

Because the `ember-basic-dropdown` trigger element has an aria-role of `button` a screenreader is unable to enter the typeahead area to do any typing.

I think it would make sense to make a `triggerRole` property in `ember-power-select` that would be passed through to the `role` attribute on the `ember-basic-dropdown` trigger.

The `` on the `ember-power-select-typeahead/trigger` should also have a role of `combobox`


Start by tracing the ember-power-select trigger and the ember-power-select-typeahead/trigger integration with ember-basic-dropdown. Verify how the trigger role is passed through and how the input is exposed to assistive technology; done means screenreader users can type in the typeahead area and the input has a combobox role.

Written by the indexing model from the issue text.