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.
