feat(input): add cursor_at_start property

Author: TomJGooding

src/textual/widgets/_input.py

@@ -468,6 +468,11 @@ def _cursor_offset(self) -> int:
             offset += 1
         return offset
 
+    @property
+    def cursor_at_start(self) -> bool:
+        """Flag to indicate if the cursor is at the start"""
+        return self.cursor_position == 0
+
     @property
     def cursor_at_end(self) -> bool:
         """Flag to indicate if the cursor is at the end"""