Add additional safety checks to insert/append card methods

[mac-gallagher]

Ensures that the value returned from dataSource.numberOfCards(in cardStack: SwipeCardStack) is correct after the update.

[codecov-commenter]

Codecov Report

Merging #94 into master will increase coverage by 0.5%.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master     #94     +/-   ##
========================================
+ Coverage    83.1%   83.6%   +0.5%     
========================================
  Files          13      13             
  Lines         687     705     +18     
========================================
+ Hits          571     590     +19     
+ Misses        116     115      -1     

Impacted Files	Coverage Δ
...Shuffle/SwipeCardStack/CardStackStateManager.swift	100.0% <100.0%> (ø)
...ources/Shuffle/SwipeCardStack/SwipeCardStack.swift	100.0% <100.0%> (+0.5%)	⬆️