Add Buffer::set_rich_text method

[tigregalis]

• Buffer::set_rich_text is a new convenience method, similar to the convenience method Buffer::set_text, for populating a buffer from an iterator of styled spans (a pair of &str and Attrs); it handles line splitting and the creation of BufferLines and AttrsLists
• The Buffer::set_text method now delegates to Buffer::set_rich_text
• The rich_text example now uses Buffer::new_empty() and Buffer::set_rich_text

use cosmic_text::{Attrs, Buffer, Family, FontSystem, Metrics, Shaping};

let mut font_system = FontSystem::new();
let mut buffer = Buffer::new_empty(Metrics::new(32.0, 44.0));
let mut buffer = buffer.borrow_with(&mut font_system);
let attrs = Attrs::new().family(Family::Serif);
buffer.set_rich_text(
    [
        ("hello, ", attrs),
        ("cosmic\ntext", attrs.family(Family::Monospace)),
    ],
    Shaping::Advanced,
);

[genusistimelord]

Very nice, this does simplify the rich example quiet a bit.