heaters: Add 'temperature_wait' status to heaters object

Report if g-code processing is delayed waiting for a heater to reach a
temperature, along with the sensor that is being checked.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/Status_Reference.md

@@ -256,6 +256,11 @@ object is available if any heater is defined):
   e.g. `["tmc2240 stepper_x"]`.  While a temperature sensor is always
   available to read, a temperature monitor may not be available and
   will return null in such case.
+- `temperature_wait`: Indicates if G-Code processing is stalled
+  waiting for a requested temperature (typically via
+  `TEMPERATURE_WAIT`, `M109`, or `M190` commands). The value will
+  contain the name of the sensor that is causing the stall or `None`
+  if no wait is in progress.
 
 ## idle_timeout