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; }
The `deprecated_format` helper is a thin wrapper to map results from
AK::vformat to a throw completion. This will let us try to throw on OOM
conditions rather than just blowing up.
Note it's called `deprecated_format` as we will likely end up adding a
method named just `format` to return `ThrowCompletionOr<String>`, when
we begin the migration from DeprecatedString->String for LibJS.
