Fixes List<T>::insert_before/after

[willnationsdev]

A separate commit to serve as a base for #13116

[akien-mga]

Could you explain what this change does and what bug it solves?

[willnationsdev]

@akien-mga Yes.

I encountered it towards the end of #10799 (with a code sample). Basically, if you use insert_before and then proceed to run a loop running from front() to back(), the chain from the preceding node into the new node is skipped, running directly to the original connection. This fixes it so that the preceding node now points to the newly inserted node, allowing for it to not be skipped. The same would hold true for loops running backwards after calling insert_after.

This may also require changes to nativescript.cpp as when I last ran a search, insert_before was getting called there as well. Whatever it was doing will now suddenly have a more complete connection, so we'll need to double check whether that changes any of the functionality (if it isn't retrieving the returned node and accessing it directly, for example).