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.

Tests/LibWeb/CMakeLists.txt

@@ -2,6 +2,7 @@ set(TEST_SOURCES
     TestCSSIDSpeed.cpp
     TestCSSPixels.cpp
     TestHTMLTokenizer.cpp
+    TestNumbers.cpp
 )
 
 foreach(source IN LISTS TEST_SOURCES)