Add example of include lua file to C to compile and run

[XuNeo]

In C

static const char lua_code_string[] = {
#include "ui.lua"
};

In lua

STRINGIZE((function()
    local root = lvgl.get_child_by_id("luaUIroot")
    root.w = lvgl.VER_RES()
    root.h = lvgl.HOR_RES()

    Button(root, {
        id = "Button in Lua",
        Label {
            text = "Hello, lua, C and lvgl!",
            align = lvgl.ALIGN_CENTER
        }
    }):center()
end)())

The additional (function() end))() is just to make lua happy with the gramma. Treat STRINGIZE is a global function in lua.