zig usage is not compliant with standard usage string

[fsaintjacques]

The following code mandates that --name and --export are provided. If they're required, they shouldn't be in the options section and be provided as arguments.

The solutions are:

• move said options to required arguments, e.g. zip build <name> <export_type> <sources>...
• provide default such that they can stay options (might be complicated with --name and multiple sources) and the user isn't forced to use them.

It creates initial churn when a new user types zig build my_file.zig.

[andrewrk]

Good call.

For some context, name and export used to be specified in the source code in a "root export" node. It was removed in f1d3381.

In the future, I expect name and export type to be specified as part of the zig build system (see #204). So, for example, these values would be specified in build.zig and the actual command one would use in practice would be simply zig build.

However, the future is not yet here, and thus you are correct that we should make this change to reduce churn.