Replace use of str::Lines with unicode-bidi-based iterator

[tigregalis]

Fixes #74

Alternative to #102 which this PR adapts the logic from.

The motivation for making this public is that any consumer of cosmic text that needs to construct buffers by pushing buffer lines needs to be able to split text into paragraphs, and this prevents re-implementation of this logic by every downstream consumer of cosmic text.

Usage is seamless:

+ use cosmic_text::BidiParagraphs;
-     for line in text.lines() {
+     for line in BidiParagraphs::new(&text) {

[tigregalis]

I've resolved the conflict.