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.
