common: add a `declare` function

We often use an fn block to declare variables on state:

fn((state) => {
   state.results = []
  return state;
});

We could introduce a declare operation, the sole purpose of which is to write a property to state:

declare('results', [])

This would be useful in https://github.com/OpenFn/water-aid/pull/31/files

The property name must be a static string (no reference, why would you declare a dynamic key? How would you read it back out again?). I suppose it could take a JSON path (in which case it has to start with $ I think?):

declare('$.results.patients', [])

The value could be a reference function. This would allow you to generate complex derived values for your state keys. It takes state as an argument and the returned value will be written to state.

Pros:

• It's offers a slightly more declarative, readable, intentional style than fn
• And actually it's much more terse too, without the faff of the nested arrow function and return statement
• It's cheap and easy to add and no-one will be upset if it's not used
• It would be quite nice in a visual job builder

Cons:

• You don't technically need it in v2, you can just do const x = 10 at the top of your job code (but only at the top!).
• You can only declare one thing at once
• Does it confuse things with fn() if the value is a function? When do you use fn and when do you use declare? I don't really think this is a problem though.

Start by reviewing the existing fn operation and the referenced water-aid pull request to understand how state declarations are currently expressed. Before implementation, resolve whether declare supports JSON paths and reference functions; done means a documented, agreed behavior for writing a statically named state property.