This commit adds a script to check for new core releases and create snapshot tags in this repository if necessary. It checks the timestamp of the core release tag and determines the last commit pushed before that timestamp.

Core was unwilling to create these tags as part of its own release process, so this is the next best solution. At some point, I'll create a Github Action to automate this, but for now, I wanted to commit the script I have been using for the past few release cycles.

Relates: #66