Split CSSStyleDeclaration into CSSStyleDeclaration and CSSStyleProperties

[asamuzaK]

Built on top of #239
Fixes #242

Spec: https://drafts.csswg.org/cssom/#the-cssstyledeclaration-interface

• Breaking change: the functionality provided by CSSStyleDeclaration is now provided by CSSStyleProperties, i.e. use new CSSStyleProperties() to create an instance.
• Breaking change: the arguments to new CSSStyleProperties() are different from the arguments to new CSSStyleDeclaration().
  • The first argument can be Window, Element or CSSStyleRule.
  • The second argument is an options object.
    • The on change callback previously received as the first argument should be included in the options object using onChange key.
      Callback is applied only when the context is Element.

      const node = document.createElement("div");
      const callback = cssText => {
        console.log(cssText);
      };
      const style = new CSSStyleProperties(node, {
        onChange: callback
      });

    • If the context is Window, element node should be included in the options object using element key.

      const node = document.createElement("div");
      const style = new CSSStyleProperties(window, {
        element: node
      });

• Switched main entry point from ./lib/CSSStyleDeclaration.js to ./lib/index.js.
• Updated dependencies and devDependencies.