Adds a defintion for essential text change

[tbostic32]

In Text content that changes automatically can be paused, stopped or hidden it was determined that we need to have essential in the expectation. However, essential is an ambiguous term, so we need to create a new definition that handles the various cases where text changing is an essential function of a page.

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[tbostic32]

@carlosapaduarte From our review we found that the applicability is somewhat hard to understand, so I would like to propose the change below:

1. Remove the no-child-changed and not-alone applicability conditions
2. Modify the change condition to only consider the text content of child text nodes (only direct children, not all descendants)

At least my understanding was that the no-child-changed condition was meant to prevent an parent element from being applicable when a the text content of a child was changed. If we only consider the text content of child text nodes then the an ancestor node will never be applicable when one of its descendants text changes.

[WilcoFiers]

Waiting for subjective applicability / state discussions to resolve before picking this back up.