In my tests this was necessary to develop Niri using non-NixOS Nix.
Otherwise Niri panics with this error message: called `Result::unwrap()`
on an `Err` value: EventLoopCreation(NotSupported(NotSupportedError)).
* add dev dependencies to flake
* parse only one default-column-width
* require exactly one action per bind, and unique keys for binds
* use proper filename for config errors if possible
* fix duplicate keybinds after invalid action, lose some sanity