Initial work on WebVTTParser class
Implemented collectDigits, collectWord, and skipWhitespace

Also moved isAsciiDigit and isHtml5Space to new Char class,
so that they are in a more central location.
Implemented WebVTTParser::parseHeader() V1, with synchronous tests

It will be simpler to make the parsers work with SynchronousBuffer
first, so for now all tests for the parsers involve SynchronousBuffer