Highlight invalid % format specifiers

It might be helpful to highlight invalid format specifiers:

    fmt.Printf("it's %.1f%", percentage)

I can't really get the regexp for that one right at the moment, and there's also the issue of wrong matches in:

    fmt.Println("100%!")

Which isn't great; not sure what a good solution for that might be. Limiting to `*f()` calls might work 🤔