git version control tool for making a local copy) of the
waveform-playlist repository on github and adding the features you need yourself. Who knows, the owner of the library might even accept your additions and merge it into the library.
If you’d like to use Tracktion engine, you’ll have to make a stand-alone application that runs outside of the browser. There are a lot of great resources for learning how to do that on the JUCE site, but if you’re new to development it may be a bit of a learning curve.
Best of luck!