Bug 1828469, part 1: Prepare `RangeBoundaryBase`  to be able to handle non-`MutationObserver` Range classes. r=masayuki

In order to support `StaticRange`s, which are not `MutationObserver`s, RangeBoundaries need to have an alternative way of ensuring that `mRef` points to the correct node.
This is now done by validating `mRef` every time `Ref()` is called using the parent and offset.
For performance reasons, this is disabled by default and should only be used for `StaticRange`s.

Differential Revision: https://phabricator.services.mozilla.com/D177892