fix(multi-select): address a11y issues

[metonym]

Supports #2172

Related #2186

This attempts to fix a11y issues reported by the IBM Accessibility Checker extension.

The default (non-filterable) MultiSelect has different semantics than the filterable variant.

This PR currently contains a fix for the nested buttons by making them siblings. Review with whitespace-hidden.

This PR is not complete:

• Non-filterable uses a button with combobox role, e.g.,:

<button type="button" aria-describedby="multiselect-helper-text-id-«r2e»" aria-activedescendant="" aria-controls="downshift-«r2g»-menu" aria-expanded="false" aria-haspopup="listbox" aria-labelledby="downshift-«r2g»-label" id="downshift-«r2g»-toggle-button" role="combobox" tabindex="0" />

• Filterable variant uses an input, and the list box field should not have any a11y attributes, similar to fix(combo-box): address a11y issues #2186

<input aria-activedescendant="" aria-autocomplete="list" aria-expanded="false" autocomplete="off" id="carbon-multiselect-example-3-input" role="combobox" aria-describedby="filterablemultiselect-helper-text-id-«r4»" aria-haspopup="listbox" value="">