Add Maven wrapper

[xenoterracide]

maven wrapper mvnw which can be generated by maven, would just make it easier to get started working on the repo. I suggest when generating it using a version that doesn't require you to commit the jar. It means that the version of maven expected to be used with the repo is something you control, and that consumers can avoid installing maven with a package manager, which means that package manager also has to install a version of java, which they might not want.

I'm not invested in this. Fortunately for me asdf can install maven now.