move tei:w and tei:pc to a proprietary namespace

[dasch124]

Currently a token is internally represented by <tei:w>, a punctuation character by <tei:pc> – which is a hard-coded assumption. What might be conceptually right can lead to unexpected behaviour in some edge cases:

• In the current state, we cannot output <tei:w> as a structure.
• We don't want to confuse such elements introduced by the tokenizer with ones already present in the markup of the source document (e.g. hy<tei:pc>-</tei:pc><lb break="no"/>phenation)

Solution: introduce <xtx:w> and <xtx:pc> elements. Moving them into the TEI namespace should be a serialization option at the end of the process.