[css-view-transitions-1][cssom] Clarify how `getComputedStyle` should be have with view-transition pseudo-elements

The current specified behavior in CSSOM re pseudo-elements is a bit vague: https://drafts.csswg.org/cssom/#extensions-to-the-window-interface

If pseudoElt is provided, is not the empty string, and starts with a colon, then:

1. Parse pseudoElt as a <pseudo-element-selector>, and let type be the result.
2. If type is failure, or is an ::slotted() or ::part() pseudo-element, let obj be null.
3. Otherwise let obj be the given pseudo-element of elt.

We should clarify what this mean for view-transition pseudo-elements?

• what happens when there is no active transition
• what happens with wildcards/classes

Current chromium implementation gives an actual value for pseudos with names but no *, and regardless of whether the transition is active. This seems a bit coincidental.

Start with the CSSOM extensions to the Window interface linked in the issue, then compare its pseudo-element rules with the listed view-transition cases. Resolve behavior for inactive transitions, wildcards, and classes, including whether the current Chromium behavior is intended. Done means the specification clearly defines these cases.