improve fix messages

[a-frantz]

This issue is the successor of #193

#260 ensured that every lint diagnostic includes a fix message, however some of them are still lacking. We should take another pass and include "corrected" code snippets wherever possible.

#264 (adds ShellCheck) uses the same fix message for every diagnostic. SC does provide its own "fix" for some diagnostics (but not all). It would be great if we could "translate" the SC fixes into our own fix messages.

[thatRichman]

I'd be happy to take a stab at this one, in the interest of finishing what I started in #264.

[a-frantz]

Sure, go right ahead! If you want to just focus on the ShellCheck fix messages, that would be fine. But we would certainly appreciate a more thorough PR that improves the other lint rule messages as well! Your call :)

I think the goal is to supply "corrected" code whenever possible. That is more straightforward for some rules than others, so the scope of this PR can be up to you.