Create a file for (S)CSS variables on install

[Spone]

When we run rails g komponent:install, let's create a file in frontend/base/variables.(s)css

When we generate a component, we should @import the variables file by default in the generated (S)CSS file. Make sure that the path is right (especially for nested components).

Or maybe we could add some configuration to be able to always import with @import "base/variables"; It's possible with https://github.com/postcss/postcss-import but we have to check with Sass.