feat: add TextArea::clear() method (#15)

Adds a clear() method that empties the entire textarea content while
integrating with the undo/redo history, so the content can be restored
with undo(). Returns true if text was deleted, false if already empty.

Ported from rhysd/tui-textarea#113.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: pm100

src/textarea.rs

@@ -1299,6 +1299,37 @@ impl<'a> TextArea<'a> {
         }
     }
 
+    /// Clear the entire content of the textarea, leaving it empty. The cursor position before calling this method
+    /// does not affect the result. This method integrates with the undo/redo history, so the content can be restored
+    /// with [`TextArea::undo`]. Returns `true` if any text was deleted, `false` if the textarea was already empty.
+    ///
+    /// ```
+    /// use ratatui_textarea::TextArea;
+    ///
+    /// let mut textarea = TextArea::from(["hello", "world"]);
+    ///
+    /// assert!(textarea.clear());
+    /// assert!(textarea.is_empty());
+    ///
+    /// textarea.undo();
+    /// assert_eq!(textarea.lines(), ["hello", "world"]);
+    /// ```
+    pub fn clear(&mut self) -> bool {
+        if self.is_empty() {
+            return false;
+        }
+        // Count all chars plus newlines between lines.
+        let total = self
+            .lines()
+            .iter()
+            .map(|l| l.chars().count())
+            .sum::<usize>()
+            + (self.lines().len() - 1);
+        self.move_cursor(CursorMove::Jump(0, 0));
+        self.delete_str(total);
+        true
+    }
+
     /// Paste a string previously deleted by [`TextArea::delete_line_by_head`], [`TextArea::delete_line_by_end`],
     /// [`TextArea::delete_word`], [`TextArea::delete_next_word`]. This method returns if some text was inserted or not
     /// in the textarea.

tests/textarea.rs

@@ -1437,6 +1437,26 @@ fn test_delete_selection_before_insert() {
     }
 }
 
+#[test]
+fn test_clear_empty() {
+    let mut t = TextArea::default();
+    assert!(!t.clear(), "clear on empty textarea should return false");
+    assert!(t.is_empty());
+}
+
+#[test]
+fn test_clear_with_undo_redo() {
+    let mut t = TextArea::from(["hello", "world"]);
+    assert!(t.clear());
+    assert!(t.is_empty());
+
+    t.undo();
+    assert_eq!(t.lines(), ["hello", "world"]);
+
+    t.redo();
+    assert!(t.is_empty());
+}
+
 #[test]
 fn test_undo_redo_stop_selection() {
     fn check(t: &mut TextArea, f: fn(&mut TextArea) -> bool) {