Clarify the usage of .paths in build.zig.zon. Follow the recommendation
of the comments to explicitly list paths by explicitly listing the paths
in the init project.
Instead of `zig init-lib` and `zig init-exe`, now there is only
`zig init`, which initializes any of the template files that do not
already exist, and makes a package that contains both an executable and
a static library. The idea is that the user can delete whatever they
don't want. In fact, I think even more things should be added to the
build.zig template.
