consider wide chars on calculating tab width

tests/mod.rs

@@ -1,2 +1,3 @@
 mod cursor;
 mod history;
+mod textarea;

tests/textarea.rs

@@ -0,0 +1,26 @@
+use tui_textarea::{CursorMove, TextArea};
+
+#[test]
+fn test_insert_soft_tab() {
+    for test in [
+        ("", 0, "    "),
+        ("a", 1, "a   "),
+        ("abcd", 4, "abcd    "),
+        ("a", 0, "    a"),
+        ("ab", 1, "a   b"),
+        ("abcdefgh", 4, "abcd    efgh"),
+        ("あ", 1, "あ  "),
+        ("🐶", 1, "🐶  "),
+        ("あ", 0, "    あ"),
+        ("あい", 1, "あ  い"),
+    ] {
+        let (input, col, expected) = test;
+        let mut t = TextArea::from([input.to_string()]);
+        t.move_cursor(CursorMove::Jump(0, col));
+        assert!(t.insert_tab(), "{:?}", test);
+        let lines = t.into_lines();
+        assert_eq!(lines.len(), 1, "{:?}, {:?}", lines, test);
+        let line = &lines[0];
+        assert_eq!(line, expected, "{:?}", test);
+    }
+}