x/website: Playground sample programs dropdown overwrites current program

[Deleplace]

What is the URL of the page with the issue?

https://go.dev/play/

Screenshot

What did you do?

Edit the default Hello World, Run, edit again, Run again, etc.

Then clicked the dropdown and chose "Conway's Game of Life".

What did you expect to see?

I wasn't sure, but I was curious.

What did you see instead?

My program was overwritten with the code of Game of Life. The "Back" button didn't work to recover my code.

Some sort of warning "Your code will be overwritten [Cancel] [OK]" would have been nice.

[toothrot]

@rsc @jamalc