Improve panic handling

[s3bba]

Today I've experienced editor crash for the first time due to panic in helix-core, lost some amount of unsaved work.
I'm trying to reproduce it, but no luck so far. I do have a panic message:

thread 'main' panicked at helix-core/src/transaction.rs:499:9:
Positions [(1125, BeforeSticky)] are out of range for changeset len 1124!

Now, regardless of if this error could have been handled more gracefully, I don't think it's good idea for editor we depend on to have no handling of unsaved buffers.

A simple solution is to add a panic hook and save any unsaved buffers to a new directory where helix is running, e.g.: .hx-crash-{timestamp} and then user can decide what do do with them.