Replace node-sass with sass.

[XhmikosR]

Node-sass is deprecated.

@jkphl there's a slight difference between node-sass and sass regarding sass.render (the sync method): https://github.com/sass/dart-sass#javascript-api

Note however that by default, renderSync() is more than twice as fast as render() due to the overhead of asynchronous callbacks. To avoid this performance hit, render() can use the fibers package to call asynchronous importers from the synchronous code path. To enable this, pass the Fiber class to the fiber option:

It seems things are OK without fibers though, so I skipped it for now.