Make Sal better configurable

[Levdbas]

This PR does four things:

• Split up scss in partials
• Make $sal-easings over writable
• Add three variables to configure Sal to your own liking
• Swap out node-sass for dart-sass.

Split up the scss in partials For better maintainability I suggest to split up the scss file in partials for ease of reading. I kept sal.scss as the entry point and created _animations.scss, _core.scss, _easings.scss and _settings.scss

Make $sal-easings over-writable You can now overwrite the easings variable. This way you are able to remove easings you don't need or add specific new ones. We might want to make the easings configurable the same way as the animations.

Add three variables to configure Sal to your own liking I have added three new variables that are also overwriteble

You are now able to control the amount of timing increments for the animation durations.

 $sal-amount-of-duration-timings: 40 !default;

You are now able to control the amount of timing increments for the animation delays.

 $sal-amount-of-delay-timings: 20 !default;

You can now remove animations you do not use from the generated scss by setting on of these animation families to false.

 $sal-animations: (
  fade: true,
  slide: true,
  zoom: true,
  flip: true
 ) !default;

Swap out node-sass for dart-sass node-sass is deprecated and it is advised to switch to Dart sass. See: https://github.com/sass/node-sass

I would love to hear your thoughts @mciastek