DMN14 — No function to add an entry to a context

Currently, it is not possible to add an entry to a given context. A context can only be declared statically.

We need to model a decision that gets a context, adds an entry, and returns the new context.

Suggestions:

• introduce a new built-in function similar to the add() function of lists but for contexts
• it is named `put` (like Java's Map#put function)
• it has three arguments: context (type: context<Any>), key (type: string), value (type: Any)
• it returns a new context that includes the new entry
• it might overwrite the value of an existing entry

put({x:1}, "y", 2)
// returns {x:1, y:2}

Introduce 3 new built-in functions for context

Introduce 3 new built-in functions for context creation, context put, context merge (of several contexts).