[stopsource.general] Restore accidentally deleted members in class definition (#7766)

These were removed in error in the application of P2300R10.

Author: timsong-cpp

source/threads.tex

@@ -919,6 +919,9 @@
     // \ref{stopsource.mem}, member functions
     void swap(stop_source&) noexcept;
 
+    stop_token get_token() const noexcept;
+    bool stop_possible() const noexcept;
+    bool stop_requested() const noexcept;
     bool request_stop() noexcept;
 
     bool operator==(const stop_source& rhs) noexcept = default;