[WC-2946] Initial setup of skiplink widget

[HedwigAR]

Pull request type

New feature (non-breaking change which adds functionality)

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Description

This PR contains a new skiplink widget, which is necessary for SC 2.4.1: Bypass Blocks .
The point of this widget is for the user to either give the ID of the main content, or to have a main tag on the page. When tabbing through the page, this widget should appear. When the user activates a link the focus should switch to the main content, skipping over repeating elements (such as the menu for example). This makes navigating the page through only keyboard easier.

What should be covered while testing?

Since this is a new widget, basically everything needs to be tested. This widget does only need to work in Mendix 11, since the option to not have auto focus on the page was introduced there.
The only thing that can't be tested yet is the design, since that's part of another story.

Expectations of this widget:

• Should be hidden when first loading the page
• Should appear when it's reached through tab, and disappear when continuing tabbing.
• Should be the first thing getting focus when tabbing
• When a link is activated, the focus should go to that link
• When a main content ID is provided, that's where the focus should go to. If this was not provided, it should look for a main element tag on the page.