rich_text and markdown widgets

[hecrj]

This PR implements support for rich text on top of cosmic-text and also introduces markdown helpers in the widget module.

Rich Text

Rich text can be created on-the-fly with the span helper and the rich_text! macro (analogous to column! and row!):

use iced::widget::{rich_text, span};
use iced::Font;

rich_text!["Hello ", span("iced").font(Font::MONOSPACE).size(20)]

Or you can store the spans in your application state and borrow them directly with the rich_text function:

rich_text(&self.my_cached_spans)

This can be useful if you are generating rich text as a result of some expensive operation, like parsing.

Markdown

Markdown support can be enabled with the new markdown feature.

Then, you can parse markdown with the parse function:

use iced::widget::markdown;

self.items = markdown::parse("# Some markdown!", Theme::TokyoNight.palette()).collect();

For now, colors need to be attached to spans during construction; therefore, a theme::Palette is needed.

Once parsed, you can render it:

use iced::widget::markdown;

markdown(&self.items)

Further configuration options will be added eventually.

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To showcase all of this, I have created a simple Markdown editor example:

Code blocks will be automatically highlighted if the highlighter feature is enabled.