refactor: Introduce struct to hold a runtime format string

This brings the format types closer to the standard library types:

* FormatStringCheck corresponds to std::basic_format_string, with
  compile-time checks done via ConstevalFormatString
* RuntimeFormat corresponds to std::runtime_format, with no compile-time
  checks done.

Also, it documents where no compile-time checks are done.

src/util/translation.h

@@ -103,7 +103,7 @@ bilingual_str format(util::BilingualFmt<sizeof...(Args)> fmt, const Args&... arg
         }
     }};
     return bilingual_str{tfm::format(fmt.original, original_arg(args)...),
-                         tfm::format(std::string{fmt.lit}, translated_arg(args)...)};
+                         tfm::format(RuntimeFormat{std::string{fmt.lit}}, translated_arg(args)...)};
 }
 } // namespace tinyformat