Add scripts to aid PRs & releases

[AndrewFerr]

This PR adds some scripts that do some maintenance-related tasks that I've been doing manually thus far, and are good to have down in writing:

• tag-release creates an appropriate tag for a new fork release, based on the release histories of the fork & upstream.
• publish-to-npm publishes the latest fork release branch to NPM & sets the correct package version while doing so.
• prepare-patch-branch creates a new branch that can be used to push commits that ought to be merged into both the fork & upstream. The branch is based on the latest common ancestor commit of the fork & upstream, which makes it easier to merge into both repos.

[Half-Shot]

Couple of things.

Generally these look like scripts scratching useful itches, so if they're working for you then we can merge them.