The UI library is in coffee script; if you want use a UI for your plugin, youre pretty much stuck using it.
(..and yes, its technically possible to do with plain js, but I challenge any of the 'but just use js' folk to link to a popular plugin, with a UI, that does)
You can call CoffeeScript functions from JavaScript and vice versa. There's no reason that you can't use the UI library from code written in JS. The fact that its written in CoffeeScript is an implementation detail
(..and yes, its technically possible to do with plain js, but I challenge any of the 'but just use js' folk to link to a popular plugin, with a UI, that does)