Implement `zig_shared_library` to generate shared objects or dynamic libraries

[Andreas Herrmann]

Replaces #16.

Bazel's builtin CC rules eventually opted to add a new cc_shared_library rule because shared library use-cases often differ from static library use-cases. So, it seems like a good starting point to implement dynamic library support in a separate rule. If we determine in future that zig_library should support both, then merging zig_shared_library into zig_library will likely be the easier migration path, than the other way around.

To not conflate this issue with #14, the first implementation should not attempt to generate or integrate with CcInfo, yet. Instead, it should just generate a .so|.dylib|.dll as a default output. Integration with the builtin C/C++ rules can be achieved as follows (sketch):

zig_shared_library(
    name = "zig-lib",
    ...
)

cc_library(
    name = "cc-lib",
    srcs = [":zig-lib"],
)

Suggested implementation steps:

• Create an end-to-end test similar to the static library test.
• Add a new zig_shared_library rule (see here and here for reference).
• Amend the common rule implementation to handle the shared library case (see here and here).
• Add a cross-platform outputs test similar to the static library test (add corresponding //zig/tests/simple-shared-library target similar to //zig/tests/simple-library).
• Handle all relevant target platforms in the implementation.
• Provide adequate rule documentation (see here for reference).
• Run bazel run //util:gazelle and bazel run //util:update to update generated targets, files, and documentation.

[Andreas Herrmann]

Closed by #103