Add support for the `wbr` element

[whisk]

Description

The library currently lacks support for the wbr element. HTML5 reference: https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-wbr-element. Note that this element has no end tag, that means it may not have children.

What to do

• [ ] Add new element function in elements.go
• [ ] Add the test case in elements_test.go