Remove panic="abort" from dev builds

[emilk]

The backtrace crate isn't compatible with panic="abort" (see docs and issue).

One great feature we have in egui is pressing down all modifier keys on your keyboard and seeing the backtrace to the widget you hover. This is awesome for when you want to change the code for some specific part of the UI. But it didn't work, at least not always.

This PR fixes it.

We still use panic="abort" in release builds, for the benefit of smaller binaries.

• See also Improve backtraces when hovering widgets with modifiers pressed emilk/egui#4696

hover-backtrace.mp4

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[teh-cmc]

The doc you linked to says:

You can remedy this, however, with -Cforce-unwind-tables as a compiler option.

Did that not work?

[emilk]

Unfortunately there is no way to add compile flags to Cargo.toml. I tried adding [build] rustflags = ["-C", "force-unwind-tables"] to .cargo/config with no effect.