Removing CurrentPosition#moving_next

For text flowing, the current position during slot positioning started to track whether if it was moving to the next line (via a `CurrentElement#moving_next`). `TextBlockFitter` used this in certain available space calculations.

Upon further investigation, since the text block has been placed, we have the values necessary in the DSL's absolutes to determine what's happening. We still use `current_position.next_line_start`, but don't require an explicit new flag.

**Note: This seems good (and simpler to boot!), but I want to do more rigorous sample testing past what I've done tonight before merging it.**

cc/ @PragTob for his excellent question about why I needed to add the `moving_next` flag.