As a Watchlist Expiry user, I want watchlist expiry support added to Edit API, so that I can watch a page when using VisualEditor, among other uses of the Edit API>
**Background:** This was split out from T248407. The edit API is what would be used when editing within VisualEditor, for instance. The code seems to trace through the `EditPage` class, and the watching happens with `WatchAction::doWatchOrUnwatch()`, which is probably being updated as part of T245565. The parameter should be called `watchlistexpiry` to be consistent with the other APIs (except for the watch API, itself). The parameter type should be "expiry", to be introduced with T248508.
**Acceptance Criteria:**
* Add watchlist expiry support to Edit API
* The parameter should be called `watchlistexpiry` to be consistent with the other APIs (except for the watch API, itself).
* The parameter type should be "expiry", to be introduced with T248508.