PDDL 2.1 Support

We have reached quite stable and mature version of the modeling capabilities for classical PDDL (1.0), the next stage is to extend towards the most used version - PDDL 2.1 (https://planning.wiki/ref/pddl21/domain).

Since PDDL 2.1 is not backwards compatible with PDDL 1.0, while both have their purpose, I suggest to create one more modeling environment for PDDL 2.1.

The main change in PDDL 2.1 comes in the definition of the action (we can consider the rest of the definition to be the same as PDDL 1.0), where the content of the action should be captured by Blockly as follows:

- **duration** takes a numeric constant or an expression with numeric fluents that evaluates to a numeric value at runtime.
- **transition**, representing the change of value, there are three of them:
-- anything -> A, **effect** what happens at the end of the action - translates to effect (at end (A))
-- A -> anything, **preconditions** what needs to hold before the action starts - translates to condition (at start (A))
-- A -> B, **persistent condition** or **transition** - translates to condition (at start (A)) and effects (at start (not (B))), (at end (A))