Enable interpretation of subtree ports as numbers.

[dsobek]

Subtree ports should not be strongly typed. This causes bugs where usage of blackboard variables become incompatible later on because of type mismatches.

This addresses https://github.com/PickNikRobotics/moveit_pro/issues/18587 on the scripting side. Instead of erroring-out, a strongly typed number summed with a non-strongly typed number should still do arithmetic. The behavior should be

ST number + ST number = number
ST number + non-ST number = number
non-ST number + non-ST number = concatenated string
^ This behavior is to prevent causing a breaking change, but we should get rid of the string concat behavior of + in the future to disambiguate things.

Examples (note that numbers in quotes are not strong types, as they would be when set in a subtree port field):

1 + 2 = 3
'1' + 2 = 3
'1' + '2' = '23'