Drop-down JS interface for `Literal` choices

E.g. 

```python
@as_function_node
def Choice(choose: Literal["a", "b", "c"]):
    chosen = choose
    return chosen
```

Should represent with a drop-down menu for choosing a, b, or c.

I swear I've seen drop down menus here before, but it might have been in Joerg's version. At any rate, the example above is not enough to give the desired behaviour on `main` right now.