Sets the first matcher for the matching plan.
Partial matchers will be run in the order in which they are added.
\Drupal\Core\Routing\InitialMatcherInterface $matcher: An initial matcher. It is responsible for its own configuration and initial route collection
\Drupal\Core\Routing\NestedMatcherInterface The current matcher.
Overrides NestedMatcherInterface::setInitialMatcher
public function setInitialMatcher(InitialMatcherInterface $initial) {
$this->initialMatcher = $initial;
return $this;
}