Fix LinkedList.insert not updating the tail when inserting after the last node

[Darkslayer3324j]

I found that LinkedList.insert(value, index) corrupts the list when the value
is inserted right after the last node (index === length): the new node is
linked after the old tail, but this.tail is not updated. The next append()
then writes to the old tail's next, overwriting the inserted node:

const list = new LinkedList();
list.append(1).append(2);
list.insert(3, 2);
list.tail.value;   // 2, should be 3
list.append(4);
list.toString();   // '1,2,4', should be '1,2,3,4'   (3 is lost)

insert already handles an index beyond the end by appending and updating
tail (the else branch), and index === 0 via prepend; only this
in-between case (walking to the last node and linking after it) missed the tail
update. I fixed it by setting this.tail = newNode when the node was linked
after the current tail.

I added a test for insert-then-append/deleteTail; I checked that it fails on
master (1 failed) and passes with the change (21 passed). eslint runs clean
via the pre-commit hook.

I found this by differential fuzzing (random operation sequences on the data
structures compared against a plain-array model); the rest of that pass (heaps,
priority queue, hash table, Fenwick tree, disjoint set, LRU caches) matched the
model.

Assisted with Claude.