feat(js): optional javascript file to adjust grid to scrollbar width

[seejamescode]

Comes after #38

Adds an optional JavaScript file included at js/yourGrid-grid.js. This will adjust the grid to not go behind browser scrollbars since different browsers handle the vw unit differently. It applies to any browser that supports CSS Variables and MutationObserver.

Thoughts, @joshblack?

[seejamescode]

Without JS:

With JS:

I have a question for you: Do you think the JavaScript is appropriate how it is or if it should be included in a module or function?

[joshblack]

Hmm, good question. Definitely would be nice to have it as a module, but given it's size maybe just have it be in UMD format would work out? Or even just an IIFE, to be honest. For example:

(function() {  
  var updateScrollbarWidth = function() {
    const scrollbarWidth =
      window.innerWidth - document.documentElement.clientWidth + "px";

    document.documentElement.style.setProperty(
      "--scrollbar-width",
      scrollbarWidth
    );
  };

  var scrollbarObserver = new MutationObserver(updateScrollbarWidth);
  scrollbarObserver.observe(document.body, {
    attributes: true,
    childList: true,
    characterData: true
  });

  updateScrollbarWidth();
})();