feat(text): implement AddAssign for Text (#1956)

This makes it possible to add a second `Text` instance to a first one using the += operator.

```rust
let mut text = Text::from("line 1");
text += Text::from("line 2");
```

Style and alignment applied to the second text is ignored (though styles and alignment of lines and spans are copied).

ratatui-core/src/text/text.rs

@@ -665,6 +665,15 @@ impl core::ops::Add<Self> for Text<'_> {
     }
 }
 
+/// Adds two `Text` together.
+///
+/// This ignores the style and alignment of the second `Text`.
+impl core::ops::AddAssign for Text<'_> {
+    fn add_assign(&mut self, rhs: Self) {
+        self.lines.extend(rhs.lines);
+    }
+}
+
 impl<'a> core::ops::AddAssign<Line<'a>> for Text<'a> {
     fn add_assign(&mut self, line: Line<'a>) {
         self.push_line(line);
@@ -940,6 +949,20 @@ mod tests {
         );
     }
 
+    #[test]
+    fn add_assign_text() {
+        let mut text = Text::raw("Red").red();
+        text += Text::raw("Blue").blue();
+        assert_eq!(
+            text,
+            Text {
+                lines: vec![Line::raw("Red"), Line::raw("Blue")],
+                style: Style::new().red(),
+                alignment: None,
+            }
+        );
+    }
+
     #[test]
     fn add_assign_line() {
         let mut text = Text::raw("Red").red();