Enhancements for Improved Web REPL Website Performance and User Experience 🏎

[sanjaiyan-dev]

Hi, this PR introduces very minor changes to the web REPL website:

1. Moving the script tag to the top with the 'defer' attribute for faster page loading and reduced render blocking (ref: https://web.dev/efficiently-load-third-party-javascript/#defer).
2. Adding the 'inputmode' attribute to the input tag for better associability.
3. Adding the 'preload' tag for critical scripts (ref: https://web.dev/preload-critical-assets/#preloading-javascript-files).
4. Adding a meta theme color with the color #555555 for a better UI.