Customize panic handling

When a panic in gdext or Rust code happens, it is currently caught and displayed as a Godot error and on the command line.

Users may want to customize this. Behaviors could be:
* Godot/console message of varying verbosity levels (message -> context -> stacktrace)
* Instantly crash (to trigger a breakpoint)
* Trigger a breakpoint without crashing (only possible with nightly [`breakpoint`](https://doc.rust-lang.org/std/intrinsics/fn.breakpoint.html) or platform specific crates/code)
* Custom handler (would need to ensure thread safety)