LibJS: Add and begin using a completion-compatible string formatter

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.

Userland/Libraries/LibJS/Runtime/StringPrototype.cpp

@@ -23,6 +23,7 @@
 #include <LibJS/Runtime/StringIterator.h>
 #include <LibJS/Runtime/StringObject.h>
 #include <LibJS/Runtime/StringPrototype.h>
+#include <LibJS/Runtime/ThrowableFormat.h>
 #include <LibJS/Runtime/ThrowableStringBuilder.h>
 #include <LibJS/Runtime/Utf16String.h>
 #include <LibJS/Runtime/Value.h>
@@ -536,8 +537,8 @@ static ThrowCompletionOr<Value> pad_string(VM& vm, Utf16String string, PadPlacem
     auto filler = filler_builder.build();
 
     auto formatted = placement == PadPlacement::Start
-        ? DeprecatedString::formatted("{}{}", filler, string.view())
-        : DeprecatedString::formatted("{}{}", string.view(), filler);
+        ? TRY(deprecated_format(vm, "{}{}", filler, string.view()))
+        : TRY(deprecated_format(vm, "{}{}", string.view(), filler));
     return PrimitiveString::create(vm, move(formatted));
 }
 

Userland/Libraries/LibJS/Runtime/ThrowableFormat.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/CheckedFormatString.h>
+#include <AK/DeprecatedString.h>
+#include <AK/Format.h>
+#include <LibJS/Runtime/Completion.h>
+#include <LibJS/Runtime/ErrorTypes.h>
+#include <LibJS/Runtime/VM.h>
+
+namespace JS {
+
+template<typename... Args>
+ThrowCompletionOr<DeprecatedString> deprecated_format(VM& vm, CheckedFormatString<Args...>&& fmtstr, Args const&... args)
+{
+    StringBuilder builder;
+    AK::VariadicFormatParams parameters { args... };
+
+    TRY_OR_THROW_OOM(vm, vformat(builder, fmtstr.view(), parameters));
+    return builder.to_deprecated_string();
+}
+
+}