Add xmake build system support - #49

Add xmake.lua as an alternative build system alongside the existing CMake setup.

Changes

• New xmake.lua at project root providing full build support
• Mirrors all CMake options: window_backend, render_backend, shared, markdown, vulkan_low_latency
• Supports xmake require package management for easy dependency resolution
• Includes install rules for headers, libraries, and pkg-config files
• Tested on Windows with MSVC 2026

Usage

# Build with defaults (GLFW + OpenGL)
xmake f -m release && xmake

# Build with SDL2
xmake f --window_backend=sdl2 && xmake

# Build shared library
xmake f --shared=y && xmake

# Install
xmake install

Package management

# Add to your xmake project
add_requires("eui-neo")

target("myapp")
    add_packages("eui-neo")

Test results (Windows/MSVC 2026)

• Build: OK
• Link: OK
• Run: OK