REVIEW ONLY: Better way to get marker at pos

DON'T MERGE THIS YET -- it breaks as many tests as if fixed, and the units tests haven't been updated yet anyway.

Reading the intent of the test, I think this is how we want to "get marker at pos", but maybe I'm misunderstanding the intent.

The existing code measures "distance" from start of range, which I don't think that we want, but it seems to work in some cases because setting the **text marker seems to be broken for empty tags** (e.g. `<meta>`, `<link>`, `<img />`). They extend to EOF.