Allow to link to all errors of a specific tool

[hasufell]

Follow-up of https://github.com/haskellfoundation/error-message-index/issues/350#issuecomment-1382672906

I'd like to have a link such as https://errors.haskell.org/messages/GHCup that shows all and only GHCup errors.

[david-christiansen]

This would be nice!

[BinderDavid]

@hasufell The site is now online at https://errors.haskell.org/ghcup/ Thanks to @haglobah who implemented this feature :)

You can also customize the ghcup specific part of the header text by modifying the message-index/ghcup/index.html file.