LibGUI: Add Ctrl-U to insert mode

While under insert mode, Ctrl-U deletes all characters between the first
non-blank character of the line and the cursor.

Implement delete_from_line_start_to_cursor() in TextEditor. Then, call
the method in VimEditingEngine via its pointer to an instance of
TextEditor.

Userland/Libraries/LibGUI/VimEditingEngine.cpp

@@ -802,6 +802,9 @@ bool VimEditingEngine::on_key_in_insert_mode(const KeyEvent& event)
         case KeyCode::Key_H:
             m_editor->delete_previous_char();
             return true;
+        case KeyCode::Key_U:
+            m_editor->delete_from_line_start_to_cursor();
+            return true;
         default:
             break;
         }