robust option list retrieval

came to mind it would be nice to be able to subclass `x-option` and rename and `x-selectmenu` would still able to retrieve the options list (by another way than `querySelectorAll('x-option')`).

also seems forwarding slots or nested slotting for options should be able to work
https://github.com/openui/open-ui/issues/565

Read the linked Open UI issue 565, then inspect how x-selectmenu currently retrieves x-option elements. Define how subclassed options and forwarded or nested slots should be discovered; done means both cases are supported without relying on querySelectorAll('x-option'), with behavior covered by the project’s relevant tests.

Written by the indexing model from the issue text.