Add Webform Matcher

I'm wondering if this should be targeting the 6.0.x branch? Usage for 8.x-5.x is much smaller (and shrinking) at this point: https://www.drupal.org/project/usage/linkit →

Uploaded a file-based patch to reference for our project.

Leaving at "Needs work" based on #32.

I've applied the patch from PR and it works fine. I'll try to find some time to add the missing test from #32.

Added a change to the WebformMatcher so it implements EntityMatcher::buildPath. This allows us to use the webform's configured alias if it's available.