SkipLink double jumps to first heading then #maincontent if disableDefaultBehaviour is not set

[waringr]

Currently if the SkipLink is included without any additional attributes, and there is an element on the page with an id of maincontent, then clicking the skip to content link causes a double jump, first to the heading, then to #maincontent.

A suggested fix would be to always call e.preventDefault, then jump to either the first heading or #maincontent based on the setting of disableDefaultBehaviour and the existence of the id, e.g.

e.preventDefault()
if(!disableDefaultBehaviour && document.getElementById('maincontent')) {
  this.focusElement(document.getElementById('maincontent'))
} else {
  this.focusElement(/* first heading*/)
}

[KaiSpencer]

Interesting discussion I will link in around the original motivation on the focus of h1.

nhsuk/nhsuk-frontend#864 (comment)