slightly improve code thanks to Go 1.18 APIs

strings.Cut makes some string handling code more intuitive.
Note that we can't use it everywhere, as some places need LastIndexByte.

Start using x/exp/slices, too, which is our first use of generics.
Note that its API is experimental and may still change,
but since we are not a library, we can control its version updates.

I also noticed that we were using TrimSpace for importcfg files.
It's actually unnecessary if we swap strings.SplitAfter for Split,
as the only whitespace present was the trailing newline.

While here, I noticed an unused copy of printfWithoutPackage.

reverse.go

@@ -162,11 +162,11 @@ One can reverse a captured panic stack trace as follows:
 			return err
 		}
 		defer f.Close()
-		any, err := reverseContent(os.Stdout, f, repl)
+		modified, err := reverseContent(os.Stdout, f, repl)
 		if err != nil {
 			return err
 		}
-		anyModified = anyModified || any
+		anyModified = anyModified || modified
 		f.Close() // since we're in a loop
 	}
 	if !anyModified {