Previously, HTMLToken would expose the Vector<Attribute> directly to
its users. In preparation for a future change, all users now use
implementation-agnostic APIs which do not expose the Vector directly.
This fixes parsing the following regular expression: /</g;
It also adds a simple script element to the HTMLTokenizer regression
test, which also contains that specific regex.
The test suite includes a few basic tests and a very crude regression
test, which just concatenates the to_string() of all tokens and checks
the String's hash to be equal. This relies on the format of
HTMLToken::to_string() to stay the same, which is not ideal.
