Add bootstrappable jars

[maxandersen]

Saw https://mill-build.org/blog/5-executable-jars.html and wondered if jbang export shouldn't have option to Exec jar this way using Java but also one using jbang to run and fetch deps using jbang.

[maxandersen]

addon for this is to have JBang-Deps: in metainf/manifest which we could download deps.

[quintesse]

Took a look at this again and although it's a neat trick, the fact that on Windows you'd need to rename the file to end with ".bat" defeats the "easy of use" IMO.