Create source release artifact

This PR is only aiming to address #11608.  It's not tackling any of the other versioning issues in the story.  It just adds a top-level "release" target which produces a "bioformats-${version}.zip" zip of the git repo using "git archive".  It also splits out the release version numbering from common.xml to a new version.xml ant script.  While this is currently just moving the one target to a new file, the intention is that this will be further modified in subsequent PRs to handle proper release versioning to address some of the other tasks in #11593 such as #11596 and #11599.

- is the "release" target name OK?
- is creating it in the project root OK, or should it go elsewhere?  I thought about jars/ along with the other artifacts, except that it's not a jar!
- is the zipfile content OK?