Avoids us having to maintain a separate hardcoded list.
This does mean we don't support parsing of `decibel` but it's not used
anywhere yet and will be supported automatically when added to
Units.json
To do this we parse the grammar supplied in `ValueTypes.json` and
generate the appropriate parsing function in `CSS::Parser::Parser`
Only a small subset of the CSS grammar (i.e. types `<foo>` and
alternatives `<foo> | <bar>`) is implemented so far, it will be
expanded in later commits.
