Add support for data-skipto parameters

[howard-e]

See #259. More specifically #259 (comment) which motivated these changes.

This will require w3c/aria-practices#2807 to be merged first. Also content/patterns/patterns.html and content/patterns/practices.html will have to be updated with the new skipto script as well in aria-practices.

Testing Instructions

Note: I created a testing branch on aria-practices, update-skipto-merged which merges 2807 and aria-practices' main, so there aren't any assumptions that have been missed following w3c/aria-practices#2702 being completed and merged which also plays a part in this work.

1. Update .gitmodules aria-practices repo branch to update-skipto-merged, like the following:

[submodule "_external/aria-practices"]
  path = _external/aria-practices
  url = https://github.com/w3c/aria-practices.git
  branch = update-skipto-merged
...

2. Run git submodule --remote --recursive to use the latest sample branch's commit instead.
3. Follow the README instructions from Install Jekyll onwards to run project as expected.
4. Observe that the updated skipto script and new parameters are being used.

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[alflennik]

Thanks for the clean solution, I like this a lot. I verified that skip-to works on all the different page types, that it's now only loaded once (awesome!), and that the styling and functionality of the different pages aren't impacted.

[css-meeting-bot]

The ARIA Authoring Practices (APG) Task Force just discussed pdating skipto.

The full IRC log of that discussion

<jugglinmike> Topic: pdating skipto
<jugglinmike> s/ pdating/ Updating/
<jugglinmike> github: https://github.com//pull/292
<jugglinmike> Matt_King: It sounds like there's some work that has to be done for which we do not currently have a pull request
<jugglinmike> Matt_King: Specifically a change in the aria-practices repository
<jugglinmike> howard-e: I suggested to do that in the same pull request which introduces the version updates
<jugglinmike> Matt_King: So you're suggesting that we add a commit to gh-2807?
<jugglinmike> howard-e: That's right--to add the "data-skipto" attributes
<jugglinmike> Matt_King: Can you make that modification?
<jugglinmike> howard-e: Sure
<jugglinmike> howard-e: It already has a couple approvals; would you like me to re-request approval from you and Jem?
<jugglinmike> Matt_King: Yes
<jugglinmike> Matt_King: What's the order of merging, here? Can you merge 292 first, allowing us to preview 2807?
<jugglinmike> howard-e: I think 292 can be merged. If we merge the main branch into 2807, we can subsequently preview that
<jugglinmike> Matt_King: Cool. This will simplify things going forward
<Jem> data-skipto="colorTheme:aria; displayOption:popup; containerElement:div"