feat: variadic inputs

Hi!

Sometimes variadic provides convenience.

For example, a custom node [`MultiAreaConditioning`](https://github.com/Davemane42/ComfyUI_Dave_CustomNode/) allows users to add 2 or more conditional inputs. If a user right-click, an option like `'insert input above'` appears.

The developer Davemane42 enables this by custom js.
https://github.com/Davemane42/ComfyUI_Dave_CustomNode/blob/main/javascript/MultiAreaConditioning.js

I think this behavior is very useful.
So natively supporting variadic would be great.

How do you think?

Thank :)