fix: multiply markWord calls

[Shipy4kaRU]

Hi! First of all - thank you for this library; it’s really one of the most capable and pleasant to use: rich feature set and a clean API.

This PR fixes a bug when markWord is used multiple times (e.g. several enhancers in a row). In your tokenization example it was failing because after the first markWord run, the leaf node in some paths became a node of type mark instead of text. On the next markWord run, leafOf(path) was called for those paths; leafOf expects a text node and throws otherwise (Invalid token path with leaf of type mark). That’s why tokenization didn’t complete in your own example. The fix: if the path’s leaf node is already non-text (e.g. a mark), we return the path unchanged and don’t call leafOf, so multiple markWord enhancers can be chained without errors

P.S: maybe it can fix #234 issue

UPD: add logs