shouldScopeCustomElement = tag => {
    if (!tagsCache.has(tag)) {
      const result = rulesObj.include.some(rule => tag.match(rule)) && !rulesObj.exclude.some(rule => tag.match(rule));
      tagsCache.set(tag, result);
    }
    return tagsCache.get(tag);
  }
Determines whether custom elements with the given tag should be scoped or not.
The tag is first matched against the "include" rules and then against the "exclude" rules and the
result is cached until new rules are set.

@public
@param tag