update remark.js version

[brettcannon]

• The bounceOut transition seems to hang (going to bounceOutUp does the transition, shows the hung bounceOut for a second, and then it clears out)
  
• bounceOutLeft leaves a margin
• bounceOutRight does the same thing
  
• rotateOut gets stuck
  
• lightSpeedOut
  

I don't know if you want these as separate bugs or if these are part of not being able to run appropriately under Linux.

Originally posted by @brettcannon in #64

[freakboy3742]

It's quite possible this is due to the version of remark not being updated to reflect recent browser changes.

However looks like remark hasn't had a formal release since 2020, and there hasn't been any code updates in 2 years... so... maybe we need to move off remark - or make the engine pluggable.