feat: make children usage easier

[ayo-run]

From discussion: https://main.elk.zone/indieweb.social/@thomasreggi/111569486541264803

Reproduction: https://gist.github.com/reggi/a4bc79ec03b9cdb1ee9fd6c61ee8e081

Found children usage is possible by using onInit hook and storing the initial children there: https://codepen.io/ayoayco-the-styleful/pen/gOqJbzW?editors=1011

[ayo-run]

Maybe a readonly this.children to access initial children will do

[ayo-run]

Also support by allowing cloneNode of template contents