Allow "pinning" a dependency, so it is not changed on global "upgrade"

[chromalchemy]

Allow :neil/pinned true on a dependency to prevend it getting bumped on neil dep upgrade

Case: I started getting errors in from nextjournal.markdown, apparently from from a java upgrade. Per an older reccomendation, if (in deps.edn) i point the dep to a Sha on a branch with a patch, it works. But every time I run the neil upgrade, it bumps that Sha to another one (presumably older, but on the main branch?), and the errors return if I dont revert the change.

[borkdude]

LGTM. @teodorlu if you want, you can take this one, else I'll look at this soon!

[teodorlu]

I'll have a look 👌