Fix dialyzer warning in Code.format_string!/2

[sabiwara]

Closes #14927

I'm starting to dislike these fixes, especially because Dialyzer is unable to follow the real types of functions like Keyword.pop. No matter what function we call, returning a keyword will be enough for Dialyzer to give up checking keys altogether.

Calling Keyword.take before each call doesn't seem great, there's an extra performance cost for no reason since these functions don't care about the extra opts.
We might as well define "open keyword" types, like we do for format_opts (| {atom(), term()}), which correspond better to the real types of these functions (the downside is we can't catch typos in opts).

[josevalim]

Good call, we can probably add the IO.inspect one here too!

[josevalim]

Please back port it too!