Allow reporting stack backtraces via Sentry

[hwittenborn]

Celeste currently includes Rust backtraces when it crashes, but these aren't very user-friendly for people who don't know much about Rust.

These backtraces also have the potential to include sensitive information (e.g. if a panic message includes a file/folder name), so we probably don't want to risk these being posted publicly. We currently mention that, but a better approach might be to ask the user if they want to upload the backtrace to a Sentry project I could set up, and then include a report ID that they can use in issues/support requests.

This would be opt-in, so it wouldn't happen by default. It would just be there as an option for users who are willing to submit them.