Split into CSSStyleDeclaration and CSSStyleProperties

[asamuzaK]

Fixes #242
Rebased #243

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 is a Window.
  • 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.

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

• Switched main entry point from ./lib/CSSStyleDeclaration.js to ./lib/index.js.
  • Exporting 3 files from index.js.
    • CSSStyleDeclaration.js
    • CSSStypeProperties.js
    • propertyList.js
      • List of supported properties and their definitions.
• Updated dependencies and devDependencies.