Add arborium-ratatui adapter crate

[fasterthanlime]

When using arborium with ratatui, you can't use spans_to_ansi because ratatui wants structured Line<'_> with Span objects that have Style attached - it does its own terminal rendering.

This means users need to reimplement the span deduplication and overlap handling logic that already exists in spans_to_themed / spans_to_ansi.

A small arborium-ratatui adapter crate could provide:

pub fn highlight_to_lines(
    highlighter: &mut Highlighter,
    theme: &Theme,
    language: &str,
    source: &str,
) -> Vec<Line<'static>>

This would handle converting arborium's themed spans to ratatui's Line/Span types with proper Style mapping, keeping the deduplication logic in one place.

Context: Implemented this manually in dibs-cli and initially got it wrong (text duplication from overlapping spans) before realizing I needed the event-based stack algorithm from spans_to_ansi.