It's no longer needed since #1260

#1320)

Termion is not supported on Windows, so we need to avoid building it.

Adds a conditional dependency to the Cargo.toml file to only include
termion when the target is not Windows. This allows contributors to
build using the `--all-features` flag on Windows rather than needing
to specify the features individually.

If the amount of characters in the screen above the viewport was greater
than u16::MAX, a multiplication would overflow. The multiply was used to
compute the maximum chunk size. The fix is to just do the multiplication
as a usize and also do the subsequent division as a usize.

There is currently another outstanding issue that limits the amount of
characters that can be inserted when calling Terminal::insert_before to
u16::MAX. However, this bug can still occur even if the viewport and the
amount of characters being inserted are both less than u16::MAX, since
it's dependant on how large the screen is above the viewport.

Fixes #1322

All urls updated to point at https://github.com/ratatui

To update your repository remotes, you can run the following commands:

```shell
git remote set-url origin https://github.com/ratatui/ratatui
```

These are simple opinionated methods for creating a terminal that is
useful to use in most apps. The new init method creates a crossterm
backend writing to stdout, enables raw mode, enters the alternate
screen, and sets a panic handler that restores the terminal on panic.

A minimal hello world now looks a bit like:

```rust
use ratatui::{
    crossterm::event::{self, Event},
    text::Text,
    Frame,
};

fn main() {
    let mut terminal = ratatui::init();
    loop {
        terminal
            .draw(|frame: &mut Frame| frame.render_widget(Text::raw("Hello World!"), frame.area()))
            .expect("Failed to draw");
        if matches!(event::read().expect("failed to read event"), Event::Key(_)) {
            break;
        }
    }
    ratatui::restore();
}
```

A type alias `DefaultTerminal` is added to represent this terminal
type and to simplify any cases where applications need to pass this
terminal around. It is equivalent to:
`Terminal<CrosstermBackend<Stdout>>`

We also added `ratatui::try_init()` and `try_restore()`, for situations
where you might want to handle initialization errors yourself instead
of letting the panic handler fire and cleanup. Simple Apps should
prefer the `init` and `restore` functions over these functions.

Corresponding functions to allow passing a `TerminalOptions` with
a `Viewport` (e.g. inline, fixed) are also available
(`init_with_options`,
and `try_init_with_options`).

The existing code to create a backend and terminal will remain and
is not deprecated by this approach. This just provides a simple one
line initialization using the common options.

---------

Co-authored-by: Orhun Parmaksız <orhunparmaksiz@gmail.com>

A vector can be compared to an array.

This new example documents the various ways to implement widgets in
Ratatui. It demonstrates how to implement the `Widget` trait on a type,
a reference, and a mutable reference. It also shows how to use the
`WidgetRef` trait to render boxed widgets.

…the viewport filled the screen (#1329)

Reimplement Terminal::insert_before. The previous implementation would
insert the new lines in chunks into the area between the top of the
screen and the top of the (new) viewport. If the viewport filled the
screen, there would be no area in which to insert lines, and the
function would crash.

The new implementation uses as much of the screen as it needs to, all
the way up to using the whole screen.

This commit:
- adds a scrollback buffer to the `TestBackend` so that tests can
inspect and assert the state of the scrollback buffer in addition to the
screen
- adds functions to `TestBackend` to assert the state of the scrollback
- adds and updates `TestBackend` tests to test the behavior of the
scrollback and the new asserting functions
- reimplements `Terminal::insert_before`, including adding two new
helper functions `Terminal::draw_lines` and `Terminal::scroll_up`.
- updates the documentation for `Terminal::insert_before` to clarify
some of the edge cases
- updates terminal tests to assert the state of the scrollback buffer
- adds a new test for the condition that causes the bug
- adds a conversion constructor `Cell::from(char)`

Fixes: #999

closes #1232 

Now we can trigger point releases by pushing a tag (follow the
instructions in `RELEASE.md`). This will create a release with generated
changelog.

There is still a lack of automation (e.g. updating `CHANGELOG.md`), but
this PR is a good start towards improving that.

🧀 

The current release steps in reference to #1337

- Bump version in `Cargo.toml`
- `git cliff -u -p CHANGELOG.md -t v0.28.1`
- Merge the PR
- `git tag v0.28.1`
- `git push origin v0.28.1`

We can probably automate away most of these with `release-plz` when it
fully supports `git-cliff`'s GitHub integration.