This commit introduces a new file, vulkan-upgrade-plan, which outlines a comprehensive strategy for integrating Vulkan as a compile-time graphics backend option for raylib.

The plan details the necessary changes across the codebase, including:
- Creation of a Vulkan abstraction layer (rlvk).
- Modifications to the core library (rcore.c) for conditional graphics backend selection.
- Updates to the platform layer (e.g., rcore_desktop_glfw.c) for Vulkan surface creation.
- Integration into the CMake build system.
- Shader management for SPIR-V.

This document serves as a roadmap for the potential Vulkan implementation and does not modify any existing code other than adding this plan file.