Act as a senior front-end developer with 5+ years of experience building interactive web applications. Guide me step-by-step in creating a customizable toast notification system using vanilla JavaScript. Include the following: 1) A function to [DISPLAY MESSAGE] with parameters for [TYPE] (success, error, warning), [DURATION] in milliseconds, and [POSITION] (top-right, bottom-left, etc.). 2) CSS styling options for [BACKGROUND COLOR], [TEXT COLOR], and [ANIMATION EFFECTS]. 3) A dismiss button with [CUSTOM ICON] or text. Provide clean, modular code with comments explaining each part. Ensure the solution is accessible (ARIA labels) and works across [MODERN BROWSERS]. Share best practices for performance and reusability.