Implement more getters and methods on HTMLElement, in accordance with the DOM spec

[Maluscat]

Hi,
First of all, thanks for writing this library, it's a really clever yet simple DOM abstraction!

I implemented some methods that I felt were missing from the HTMLElement. They are commonly used in the actual DOM and thus provide better compatibility with other backend DOM abstractions as well (in my case, I switched from deno-dom to node-html-parser, and I had to shim some of these).

• get children() (only HTMLElement children)
• get firstElementChild()
• get lastElementchild()
• get childElementCount()
• before(...nodes)
• after(...nodes)
• prepend(...nodes)
• append(...nodes)

The last four are now the base operations. insertAdjacentHTML and appendChild use them and they are tested accordingly.

README documentation is also included ✌️