Add support for watchlist labels to the index.php?action=watch page (i.e. WatchAction).
This will make adding labels possible for users who open the watchstar or link in a new tab, use a skin that doesn't support the watchstar menu, or aren't using JavaScript. It matches the existing functionality for watching and for adding expiry times.
The existing form looks like this (at e.g. https://example.org/w/index.php?title=Lorem_ipsum&action=watch):
This should honour the $wgEnableWatchlistLabels feature flag (unless that's already been removed in T417359).
The action=watch page doesn't make it very clear when it's being used to change an existing watchlist item, but that's an existing issue and not one that this task will sort out.