Command block that restores scroll offsets and selection like Emac Lisp save-excursion

> Note: This may not be needed in practice at all. We have to test this.

-------------

One feature that Emacs Lisp provides is to jump around in a buffer and read and write text, then restore the context exactly as it was before so that the user doesn't perceive anything. This is called `save-excursion`.

```swift
// Imagine we're in the middle of a long document here, like
// line 500 of 1000.

SaveExcursion {
Select(LineRange(at: 0)) {
// ... do stuff on the first line ...
}
Select(LineRange(at: buffer.endLocation)) {
// ... do stuff on the last line line ...
}
}

// At this point, we're at line 500 again and the scroll offset is the same as before
```

Unlike our `Buffer.character(at:)`, Emacs buffers can't access substrings without moving them into view. There, the buffer is the model, and moving the reading head also moves the scrolled viewport.

`NSTextView` can show similar behavior when you change characters in one place and attributes in a larger place and the effective range is enlargened; then the `NSTextStorage.processEditing()` run will move the insertion point:

Source and fix:

`UITextView` doesn't have this problem.

まずこの挙動が必要かどうかをテストし、次に issue で説明されている Buffer.character(at:) と NSTextStorage.processEditing() の挙動を調べます。実装方針を選ぶ前に、NSTextView と UITextView のケースを比較します。完了とは、この挙動が必要な場合に、コマンドがバッファを変更しながら以前の選択範囲とスクロールオフセットを復元できることを意味します。

索引モデルが issue の本文から書いたものです。