feat(nodes): add basic dictionary manipulation nodes

[bigcat88]

Since using LLMs inside workflows is becoming increasingly popular, I propose adding basic nodes for working with dictionaries to make it easier to parse JSON responses from LLMs.

For basic usage, introducing a new type DICT and four small nodes is sufficient. Future functionality can be extended as needed. This will allow users to:

• create a dictionary
• convert a string to a dictionary
• merge two dictionaries
• get a value from a dictionary

Each node that may encounter an execution error includes an optional fallback parameter.