Add focus() method to nuxeo-selectivity

nuxeo-input has a focus() method which can be used to focus the widget upon opening an Edit layout while nuxeo-selectivity does not. It is possible to manually focus the element by targeting its embedded input element as below:

const input = selectivityComponent.s2.$.input.querySelector('input');
input.focus();

nuxeo-selectivity should have a focus() method that implements the above to match nuxeo-input's functionality.