Define maximum length and handling of CharacterData nodes

[annevk]

Currently we pretend that all CharacterData nodes (Text, CDATASection, ProcessingInstruction, and Comment) can have infinite length. In practice however it seems the upper limit has to be JavaScript's String's maximum length, which is 2⁵³ − 1.

Should we standardize on that and attempt to deal with the consequences for wholeText, normalize(), etc?

cc @rniwa @smaug---- @mfreed7