proof_of_work 1.0.0-alpha4

Fourth alpha release of Proof of Work, focused on the keyboard and screen reader experience of the Shape matching challenge.

Added

• Shape matching: keyboard and screen reader support with a small script. Arrow keys, Home and End move between the options of the current shape without selecting them. After a choice the focus moves to
  the next shape, or to Continue after the last one, and Start over returns the focus to the first option. A live region announces the current shape.

Changed

• Shape matching: the link to continue with a waiting time is shown above the challenge instead of below it, so screen reader users hear it first.
• Shape matching: a chosen shape can be unchecked again; that shape becomes the current one and the other answers stay in place under their shape.
• Shape matching: Continue is only shown once every shape is answered, also without JavaScript, next to Start over.
• Shape matching: the fallback for browsers without :has() is removed; all current browsers support it.

The challenge keeps working without JavaScript; the script only improves keyboard and screen reader use.

This is an alpha release: APIs and configuration may still change. Feedback in the issue queue is welcome.