`DeprecatedString::to_int` calls `StringUtils::convert_to_int`
internally. However, the integer parsing is not done in an HTML
spec-compliant way. For example, `colspan="2;"` is valid according to
the spec. But, with the current implementation, we will fail to parse
"2;", and instead fall back to using 1 as the colspan value.
This patch changes the `HTMLTableCellElement::col_span` and
`HTMLTableCellElement::row_span` methods to use the
`Web::HTML::parse_non_negative_integer` function that will parse the
attribute value in an HTML spec-compliant way.
