Add example of mixed UI programming with Lua and C

[XuNeo]

With this update, you can now design your UI in Lua and seamlessly integrate it with C, merging the simplicity of luavgl with the power of C.

Say goodbye to hundreds of lines of style settings—luavgl makes UI creation a breeze!

static const char lua_code_string[] = STRINGIZE(
    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()
);

static int embed_lua_in_c(lua_State *L)
{
  /* We create a root obj in C and create other UIs in lua. */
  lv_obj_t *root = lv_obj_create(lv_screen_active());
  luavgl_add_lobj(L, root)->lua_created = false;
  lv_obj_set_id(root, lv_strdup("luaUIroot"));
  luaL_dostring(L, lua_code_string);
  return 0;
}