Move focus starting point on anchor scroll

[daun]

Description

• Solve Feature: Move focus when handling same-page links #43
• Move focus to right before an anchor on anchor scroll
• Requires an ugly invisible-element workaround due to Safari
• Switch content focus to new method as well, remove notes about no-longer-required styling
• Works great in major browsers, tested in local playground
• Caveat: resets focus to body on screen readers, so not ready to merge yet

Checks

• The PR is submitted to the master branch
• The code was linted before pushing (npm run lint)
• All tests are passing (npm run test)
• New or updated tests are included
• The documentation was updated as required

[daun]

Closed in favor of #55