AK: Add support for "debug only" formatters

These are formatters that can only be used with debug print
functions, such as dbgln(). Currently this is limited to
Formatter<ErrorOr<T>>. With this you can still debug log ErrorOr
values (good for debugging), but trying to use them in any
String::formatted() call will fail (which prevents .to_string()
errors with the new failable strings being ignored).

You make a formatter debug only by adding a constexpr method like:
static constexpr bool is_debug_only() { return true; }

Userland/Libraries/LibJS/Runtime/ThrowableFormat.h

@@ -19,7 +19,7 @@ template<typename... Args>
 ThrowCompletionOr<DeprecatedString> deprecated_format(VM& vm, CheckedFormatString<Args...>&& fmtstr, Args const&... args)
 {
     StringBuilder builder;
-    AK::VariadicFormatParams parameters { args... };
+    AK::VariadicFormatParams<AK::AllowDebugOnlyFormatters::No, Args...> parameters { args... };
 
     TRY_OR_THROW_OOM(vm, vformat(builder, fmtstr.view(), parameters));
     return builder.to_deprecated_string();