LibWeb: Implement spec-compliant integer parsing

We have code inside LibWeb that uses the
`AK::StringUtils::convert_to_uint`and `AK::StringUtils::convert_to_int`
methods for parsing integers. This works well for the most part, but
according to the spec, trailing characters are allowed and should be
ignored, but this is not how the `StringUtil` methods are implemented.

This patch adds two new methods named `parse_integer` and
`parse_non_negative_integer` inside the `Web::HTML` namespace that uses
`StringUtils` under the hood but adds a bit more logic to make it spec
compliant.

Userland/Libraries/LibWeb/HTML/Numbers.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2023, Jonatan Klemets <jonatan.r.klemets@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+#include <AK/String.h>
+
+namespace Web::HTML {
+
+Optional<i32> parse_integer(StringView string);
+
+Optional<u32> parse_non_negative_integer(StringView string);
+
+}