Don't use JS-based prompts

[atjn]

JavaScript-based prompts like alert, prompt and confirm are quick and easy to use, but are generally not a recommended way to interact with the user.

We cannot control how the browser displays them, or if the browser displays them at all. For example, a common annoyance with them is that the user can't close the browser while a prompt is being shown. They are also known to have accessibility issues. Another factor for us is that Tauri doesn't have full support for all prompt types, and we would like to only use browser features that we know works correctly in Tauri as well.

We should therefore build a few Svelte components that work on top of the standard modal component we are currently working on. We can make whatever prompt types that are necessary, and can then replace all native JS prompts with calls to open these custom modals. The API to do so can be modelled after the native JS API's if that is what we find to be the best developer experience.