I've introduced the foundational infrastructure for a Vulkan rendering backend in raylib.

Key changes:
- CMake:
    - I added a `SUPPORT_VULKAN` option (default OFF) to enable Vulkan.
    - It finds the Vulkan SDK and links appropriate libraries when enabled.
    - I defined `CF_VULKAN_` (0 by default, 1 if Vulkan enabled) and `GRAPHICS_API_VULKAN` preprocessor macros.
    - I updated `LibraryConfigurations.cmake` and `src/CMakeLists.txt` to handle Vulkan as a graphics API.
- Vulkan Abstraction Layer:
    - I created `src/rlvk.h` and `src/rlvk.c` with stub implementations for Vulkan rendering functions (e.g., `rlvkInit`, `rlvkClose`, drawing functions). These are compiled only when `SUPPORT_VULKAN` is ON.
- Core Integration:
    - `rlgl.h` and `rcore.c` now have conditional code paths for `GRAPHICS_API_VULKAN`.
    - `InitWindow` and `CloseWindow` in `rcore.c` call Vulkan-specific platform and backend initialization/deinitialization stubs.
- Platform Layer (GLFW):
    - `src/platforms/rcore_desktop_glfw.c` includes stubbed `InitPlatformVulkan` and `ClosePlatformVulkan` to set up GLFW for a Vulkan context (actual Vulkan instance/surface creation is stubbed).

This commit establishes the necessary build system changes and C code structure to allow for the incremental implementation of the Vulkan renderer. Currently, enabling Vulkan will compile the stubs, allowing your application to run but without actual Vulkan rendering. The `CF_VULKAN_` flag controls the compilation and selection of Vulkan as the default renderer when active.