Fix Thermal Runaway false-alarm in M303, add HeaterWatch::check (#21743)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/src/module/temperature.h

@@ -233,6 +233,8 @@ struct HeaterWatch {
   inline bool elapsed(const millis_t &ms) { return next_ms && ELAPSED(ms, next_ms); }
   inline bool elapsed() { return elapsed(millis()); }
 
+  inline bool check(const celsius_t curr) { return curr >= target; }
+
   inline void restart(const celsius_t curr, const celsius_t tgt) {
     if (tgt) {
       const celsius_t newtarget = curr + INCREASE;