Since `none-ls` has removed their `shellcheck` built-in. This actually
makes the diagnostics more robust to POSIX/non-POSIX scripts (the LSP
server detects it at runtime, which is more robust than the `ftdetect`
scripts).
Nice bonus: the shellcheck code is shown in the diagnostics message
without any configuration!
I'm not sure if I can configure `avoid-nullary-conditions` -- though it
seems like this check is broken at the moment (I couldn't get it to
trigger during my tests).
