Add OSX + Metal Backend

[jrachele]

Hello,

I'm currently writing a Zig project that requires me to mess with Cocoa directly, so I decided to add the OSX/Metal backends from ImGui to this project. I took these from the v1.91.0 tagged backends: https://github.com/ocornut/imgui/blob/v1.91.0/backends

Zig changes:

• Add backend_osx.zig, and backend_osx_metal.zig, which follow the same patterns as the other backends
• Add objective C flags to build script to disable various warnings that Zig treats as errors

ImGui changes:

• extern "C" {} wrappings around the exposed Objective C functions in both osx and metal backends.
• Removed GameController support from imgui_impl_metal.mm as it was unhappy on macOS 10.15