Code which previously called `buf.get(x, y)` or `buf.get_mut(x, y)`
should now use index operators, or be transitioned to `buff.cell()` or
`buf.cell_mut()` for safe access that avoids panics by returning
`Option<&Cell>` and `Option<&mut Cell>`.
The new methods accept `Into<Position>` instead of `x` and `y`
coordinates, which makes them more ergonomic to use.
```rust
let mut buffer = Buffer::empty(Rect::new(0, 0, 10, 10));
let cell = buf[(0, 0)];
let cell = buf[Position::new(0, 0)];
let symbol = buf.cell((0, 0)).map(|cell| cell.symbol());
let symbol = buf.cell(Position::new(0, 0)).map(|cell| cell.symbol());
buf[(0, 0)].set_symbol("🐀");
buf[Position::new(0, 0)].set_symbol("🐀");
buf.cell_mut((0, 0)).map(|cell| cell.set_symbol("🐀"));
buf.cell_mut(Position::new(0, 0)).map(|cell| cell.set_symbol("🐀"));
```
The existing `get()` and `get_mut()` methods are marked as deprecated.
These are fairly widely used and we will leave these methods around on
the buffer for a longer time than our normal deprecation approach (2
major release)
Addresses part of: https://github.com/ratatui-org/ratatui/issues/1011
---------
Co-authored-by: EdJoPaTo <rfc-conform-git-commit-email@funny-long-domain-label-everyone-hates-as-it-is-too-long.edjopato.de>
