Dear ImGui

Dear ImGui does not have native CMake support, and while adding the sources to the executable is an option, we will add it as an external library target: imgui to isolate it (and compile warnings etc) from our own code. This requires some changes to the ext target structure, since imgui will itself need to link to GLFW and Vulkan-Headers, have VK_NO_PROTOTYPES defined, etc. learn-vk-ext then links to imgui and any other libraries (currently only glm). We are using Dear ImGui v1.91.9, which has decent support for Dynamic Rendering.