Support query templates in `default` field property

Example:

default: "{​{​ page.title }}"

I recently needed this and was surprised to learn that it’s not currently supported.

There was a PR for this that unfortunately got closed and not merged: https://github.com/getkirby/kirby/pull/2815.

Relevant thread: https://forum.getkirby.com/t/kirby3-blueprint-text-field-dynamic-default-value-from-page-title/18898/29