Decide a package manager for website

[GomezIvann]

We have maintained both yarn and npm as our package managers on the Halstack website for a while. This can be inconvenient:

Since both of them do the same, I think we need to decide and stick with just one manager to avoid future issues with our dependencies.

[kukoc]

We will go for npm as package manager. NPM it's widely used since it's installed by default along with Node