support saving and loading checkpoints

[philschatz]

Some games use checkpoints rather than levels to save the players' progress. These are usually large games that require the player to move between multiple screens in order to complete puzzles. This PR adds support for saving and loading those checkpoints 🎉

Refs #104 and #107

TODO

• store each checkpoint in a separate key in localStorage (& clear them when the level changes)

Screencap showing that progress is saved by reloading the page a couple of times:

/cc @lwtchu . This PR will likely also fix the other 0% and 100% complete bugs 😄

[codecov]

Codecov Report

Merging #113 into master will increase coverage by 0.03%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
+ Coverage   47.99%   48.03%   +0.03%     
==========================================
  Files         144      144              
  Lines       11840    11848       +8     
  Branches      854      856       +2     
==========================================
+ Hits         5683     5691       +8     
  Misses       5985     5985              
  Partials      172      172

Impacted Files	Coverage Δ
src/replaySolutions/helper.ts	93.05% <100%> (ø)	⬆️
src/util.ts	90.54% <100%> (ø)	⬆️
src/cli/playGame.ts	36.17% <100%> (ø)	⬆️
src/engine.ts	88.79% <80%> (+0.18%)	⬆️

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