Introduce a workflow updating the wishlist leaderboards (#5085)

- Closes https://github.com/paritytech/eng-automation/issues/11

The workflow periodically updates the leaderboards of the wishlist
issues: https://github.com/paritytech/polkadot-sdk/issues/3900 and
https://github.com/paritytech/polkadot-sdk/issues/3901

The code is adopted from
[here](https://github.com/kianenigma/wishlist-tracker), with slight
modifications.

Previously, the score could be increased by the same person adding
different reactions. Also, some wishes have a score of 0 - even thought
there is a wish for them, because the author was not counted.

Now, the score is a unique count of upvoters of the desired issue,
upvoters of the wish comment, and the author of the wish comment.

I changed the format to include the `Last updated:` at the bottom - it
will be automatically updated.