Adds the Picojson interface target, C99 Md4c from md4c/src and NanovgVk
from nanovg_vulkan/src (nanovg.c plus the nanovg_vk.c backend). ImGui
compiles imgui_impl_vulkan alongside the existing GLFW and OpenGL
backends, so current OpenGL users keep linking. Desktop and test libraries
stay enabled; the port's MODULES_BUILD_DESKTOP and MODULES_BUILD_TESTS
gates are not used.
md4c and picojson are pinned, unmodified mirrors of mity/md4c (f9e5eb2)
and kazuho/picojson (111c9be). nanovg_vulkan points at the modules-vulkan
branch of a fork of SubiyaCryolite/nanovg_vulkan, which carries the
backend fixes made for Modules as reviewable commits on top of upstream.