`zig_library` should support generating a dynamic C library

[aherrmann]

Consider whether this should be part of zig_library or a dedicated zig_shared_library instead.
Consider how zig_library should behave compared to cc_library's behavior depending on the linkstatic attribute and fully_static_link feature.
Consider what take-away from the transitive C/C++ libraries discussion should be applied here.

[aherrmann]

Closed in favor of #102.