Add unwrap widget value support

[huchenlei]

Currently if the frontend sets a list as widget value, it will be interpreted as a link between 2 nodes. This PR adds an unwrap mechanism to fix this issue. The frontend will wrap the widget value in a dict with reserved key __value__, e.g. {"__value__": [1, 2, 3]}, and the execution engine unwraps it to restore the internal content.

[christian-byrne]

Would it be possible to instead change prompt validation to allow lists?

[huchenlei]

Would it be possible to instead change prompt validation to allow lists?

What do you mean change prompt validation?

[christian-byrne]

What do you mean change prompt validation?

Change the validation logic of validate_prompt to accept list inputs which can be marked e.g., using LIST input type.

[huchenlei]

What do you mean change prompt validation?

Change the validation logic of validate_prompt to accept list inputs which can be marked e.g., using LIST input type.

I think binding the array value type to a LIST input type is not ideal as that restricts the space for custom nodes. For example, custom nodes author might want to define VEC2, VEC4 for arrays of certain length, instead of just using generic LIST type.