Remove first parameter of `Element` constructor

[willeastcott]

Currently, the Element constructor takes 2 arguments:

• A DOM element (or string representing a DOM element that can be passed to document.createElement).
• An options object. The options object allows for a dom property to be set which duplicates what can be specified by the first argument.

The Element constructor signature differs from all of its subclasses in having this first element. I propose we:

• Drop this first argument and instead rely solely on the dom property of the options object (which defaults to div if not specified).
• Bump major version number to 4.0.0 since this is a breaking API change.