implement Scrolling enum to specify scrolling more flexibly

this is a breaking change. next release should be v0.2.0.

README.md

@@ -415,7 +415,7 @@ notify how to move the cursor.
 | `textarea.set_search_pattern(pattern)`               | Set a pattern for text search                |
 | `textarea.search_forward(match_cursor)`              | Move cursor to next match of text search     |
 | `textarea.search_back(match_cursor)`                 | Move cursor to previous match of text search |
-| `textarea.scroll(rows, cols)`                        | Scroll the viewport                          |
+| `textarea.scroll(scrolling)`                         | Scroll the viewport                          |
 
 To define your own key mappings, simply call the above methods in your code instead of `TextArea::input()` method. The
 following example defines modal key mappings like Vim.
@@ -468,8 +468,8 @@ loop {
             Input { key: Key::Esc, .. } => textarea.set_search_pattern("").unwrap(),
             Input { key: Key::Char('n'), .. } => textarea.search_forward(),
             Input { key: Key::Char('N'), .. } => textarea.search_back(),
-            Input { key: Key::Char('e'), ctrl: true .. } => textarea.scroll(1, 0),
-            Input { key: Key::Char('y'), ctrl: true .. } => textarea.scroll(-1, 0),
+            Input { key: Key::Char('e'), ctrl: true .. } => textarea.scroll((1, 0)),
+            Input { key: Key::Char('y'), ctrl: true .. } => textarea.scroll((-1, 0)),
             _ => {},
         },
         Mode::Insert => match read()?.into() {