Bug 1810403: Allow `nsRange`s to be in multiple `Selection`s. r=masayuki

The Custom Highlight API allows a use case where a `Range` of a `Highlight`
is also used as `Selection`. Due to the decision to use the `Selection` mechanism
to display `Highlight`s, a `Range` can be part of several `Selection`s.
Since the `Range` has a pointer to its associated `Selection`
to notify about changes, this must be adapted to allow several `Selections`.

As a tradeoff of performance and memory usage, the `Selection`s are stored
as `mozilla::LinkedList`. A helper class `mozilla::SelectionListWrapper`
was implemented to allow `Selection`s to be in multiple of these lists
and without having to be derived from `LinkedListElement<T>`.

To simplify usage of the list, the use case  "does this range belong to Selection x?"
is wrapped into the convenience method`IsInSelection(Selection&)`;
The method previously named like this was renamed to `IsInAnySelection()`
to be named more precisely.

Registering and unregistering of the closest common inclusive ancestor
of the `Range` is done when the first `Selection` is registered and
the last `Selection` is unregistered.

Differential Revision: https://phabricator.services.mozilla.com/D169597