Add API for getting which visible paragraph index the caret is on

[JordanMartinez]

currentParagraph is used to get the caret's paragraph index in terms of the total paragraphs, but now that #501 has been merged, there's not a counterpart for determining the caret's paragraph index in terms of the visible paragraphs. It should be Optional<Integer> currentVisibleParagraph

[JordanMartinez]

This is also something that can be done via #600.