Support formatted text

[noelwelsh]

We need a way to support formatted text. That is, text with associated styling that can change per character. Something like fansi. This should be fairly simple to implement.

[noelwelsh]

This also surfaced in the ui module: text.Line carries no styling, so the Text component applies one set of CellProps per line. The equalizer demo (interactiveDemo in ui/native/.../Example.scala) works around it by exploding each gradient bar into one single-cell Text per cell — fine for a 16-cell demo, doesn't scale. Whatever shape formatted text takes (e.g. a line of runs: Vector[(Line, CellProps)]), the ui buffer rendering should consume it directly.