Add base concept for search role

[scottaohara]

As the search element has landed in the HTML spec, now the search role can reference it as its HTML 'base concept', adding this row to the search role's characteristics table

This would be a good first issue for someone who wants to get a quick PR into the ARIA spec. :)

[thibaudcolas]

I’m interested in opportunities for contributing to specifications! Will give this a go.