Add flexible option for size to support flex width

[hellokaton]

I was reviewing the Turnstile widget documentation and noticed that the "size" option currently supports only a fixed set of values. I would like to suggest adding a "Flexible" option for size, which would allow the widget to adapt to elastic or dynamic widths.

This would be really helpful for responsive designs where the widget needs to scale based on the container's width. You can refer to the official documentation for more details: Cloudflare Turnstile Widget Documentation.

Thank you for considering this enhancement!