Introduce constants for #cache keys

This issue is being reviewed by the kind folks in Slack, #needs-review-queue-initiative. We are working to keep the size of Needs Review queue [2700+ issues] to around 400 (1 month or less), following Review a patch or merge request → as a guide.

Love the idea! Should/could we add a simple test coverage to show them in use

Also tagged for a change record as others may be excited to use.

Have added test coverage. Changing to 'Needs review'.

Seems to have a failure.

Also will need the CR

Please review the new change record → .

Tests are all green.

Looks good, approved.

This is missing support for $element['#cache']['keys'], some comments seem to have snuck through the sniffers. I'm not sure how much value the test adds, to be honest. We're simply testing for the value of a constant, I don't see the point in that.

I'm on the fence regarding removing the test coverage, because it's really not testing anything other than "is constant X value Y". So if you ask me I'd remove them. But let's leave that for core committers to decide.