feat: add ergonomic methods for layouting Rects (#1909)

This commit introduces new methods for the `Rect` struct that simplify
the process of splitting a `Rect` into sub-rects according to a given
`Layout`. By putting these methods on the `Rect` struct, we make it a
bit more natural that a layout is applied to the `Rect` itself, rather
than passing a `Rect` to the `Layout` struct to be split.

Adds:
- `Rect::layout` and `Rect::try_layout` methods that allow splitting a
  `Rect` into an array of sub-rects according to a given `Layout`.
- `Rect::layout_vec` method that returns a `Vec` of sub-rects.
- `Layout::try_areas` method that returns an array of sub-rects, with
  compile-time checks for the number of constraints. This is added
  mainly for consistency with the new `Rect` methods.

```rust
use ratatui_core::layout::{Layout, Constraint, Rect};
let area = Rect::new(0, 0, 10, 10);
let layout = Layout::vertical([Constraint::Fill(1); 2]);

// Rect::layout() infers the number of constraints at compile time:
let [top, main] = area.layout(&layout);

// Rect::try_layout() and Layout::try_areas() do the same, but return a
// Result:
let [top, main] = area.try_layout(&layout)?;
let [top, main] = layout.try_areas(area)?;

// Rect::layout_vec() returns a Vec of sub-rects:
let areas_vec = area.layout_vec(&layout);

// you can also explicitly specify the number of constraints:
let areas = area.layout::<2>(&layout);
let areas = area.try_layout::<2>(&layout)?;
let areas = layout.try_areas::<2>(area)?;
```

examples/apps/async-github/src/main.rs

@@ -78,8 +78,8 @@ impl App {
     }
 
     fn render(&self, frame: &mut Frame) {
-        let vertical = Layout::vertical([Constraint::Length(1), Constraint::Fill(1)]);
-        let [title_area, body_area] = vertical.areas(frame.area());
+        let layout = Layout::vertical([Constraint::Length(1), Constraint::Fill(1)]);
+        let [title_area, body_area] = frame.area().layout(&layout);
         let title = Line::from("Ratatui async example").centered().bold();
         frame.render_widget(title, title_area);
         frame.render_widget(&self.pull_requests, body_area);