Split checker implementation

[kanak8278]

Nl2DSL should only contain validation and checks which makes sure the DSL conforms the required structure.

1. Validate Structure for each task type. (Check for keys, transitions, e)
2. Check if the variables used in the task is defined in Pydantic Class.
3. Check for the transition state exists, if doesn't exist set it None.

All errors should be caught in NL2DSL.