Prepare and export CSSStyleProperties

[asamuzaK]

Related #235

[asamuzaK]

Spec:

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

Ref:

• [cssom] The way CSSStyleDeclaration exposes properties is not ideal. w3c/csswg-drafts#5649
• [cssom] [css-fonts] [css-page] Sort out properties and descriptors. w3c/csswg-drafts#9686
• https://bugzilla.mozilla.org/show_bug.cgi?id=1919582
• https://bugs.webkit.org/show_bug.cgi?id=238273