Make it dead easy to use the Scala Toolkit

[SethTisue]

audience suggestion at Scala Days Madrid after @szymon-rd's talk about the Toolkit

currently I can type "toolkit" into the Scastie search box and the Scala toolkit does come up:

though the Typelevel one comes first. perhaps we should have a dedicated checkbox for adding the Toolkit, without having to search for it

this might overlap a bit with #919 since Scala-CLI supports //> using toolkit latest and sbt doesn't have that. but even having to know how to write that magic comment is an obstacle, just like having to use the search box is an obstacle, so I think this could go forward regardless.