Add validation to path aliases

Technically this is a Drupal core bug, isn't it? 🤔

(I'm fine with building it in XB first, of course.)

Well, the validation happens, but it happens in the PathAlias entity instead of the PathItem. Is there a strict need to effectively validate twice, or do we only need it in PathItem because of special requirements in XB?

There does seem to be alias validation, when I enter invalid path alias, it does report an error:

BTW there is also the problem of path aliases (new or changed) not being saved :/ 🐛 Page path alias is not actually saved Active