Add args() accessor back to fmt::format_context (#4310)

Add args() accessor back to fmt::format_context
Add test that would fail to compile if you can't create a fmt::format_context from another fmt::format_context

include/fmt/base.h

@@ -2656,6 +2656,7 @@ class context {
   FMT_CONSTEXPR auto arg_id(string_view name) const -> int {
     return args_.get_id(name);
   }
+  auto args() const -> const format_args& { return args_; }
 
   // Returns an iterator to the beginning of the output range.
   FMT_CONSTEXPR auto out() const -> iterator { return out_; }