Add support for generating QR codes when sharing

The slides for the closing keynote at this year's CPPCon include QR codes linking to runnable code on the Godbolt Compiler Explorer: http://vandevoorde.com/CppCon2024.pdf

It would be very cool to be able to easily do the same with the Rust Playground, by adding an action to the Share menu to generate a QR code image which the user can download and which links to the code.