Introduce OD_Element class

[westonruter]

Important

This is a follow up to #1373. Do not merge this before that is merged into trunk.

See https://github.com/WordPress/performance/pull/1373/files#r1792652840:

See note below on the get_all_denormalized_elements() method that I'd like to instead have $element be an OD_Element class instance which has a $url_metric property to access the OD_URL_Metric instance if is a part of. In the same way, the OD_URL_Metric class can have a $group property to indicate the OD_URL_Metric_Group it is a part of. This would eliminate the need to pass back an array of tuples and instead. So instead, to get the $group this code could do $element->url_metric->group.

[westonruter]

I don't like that $url_metric->get( 'elements' ) returns an array of arrays (the underlying values), whereas $url_metric->get_elements() returns the data wrapped in OD_Element objects.

[westonruter]

I don't really like the get_all_elements name. It's more like get_elements_grouped_by_xpath.

[westonruter]

I implemented ArrayAccess here for back-compat with existing code accessing the offsets. But for the way we access data, perhaps we should have accessor get methods similar to what is done for OD_URL_Metric. So this would have:

• is_lcp()
• is_lcp_candidate()
• get_xpath()
• get_intersection_ratio()
• get_intersection_rect()
• get_bounding_client_rect()

As well as a get() method which would return arbitrary values, especially those which are from an extension-extended schema, like $element->get( 'resizedBoundingClientRect ) in Embed Optimizer.