reactive.Value: add freeze() method

Author: wch

shiny/reactive/_reactives.py

@@ -89,6 +89,10 @@ def is_set(self) -> bool:
         self._dependents.register()
         return not isinstance(self._value, MISSING_TYPE)
 
+    # Like unset(), except that this does not invalidate dependents.
+    def freeze(self) -> None:
+        self._value = MISSING
+
 
 # ==============================================================================
 # Calc