Basic UDF support for Node API

[hannes]

This PR adds basic JS UDFs for the Node API. Here is a minimal example:

db = new duckdb.Database(':memory:');
db.register("my_udf", "integer", (x) => x+1);
db.all("select my_udf(42) v", function(err, rows) {
   console.log(rows[0]);
});
db.unregister("my_udf");

[samansmink]

My node API experience is very limited but I added two small comments. The rest looks good to me!

[hannes]

I seem to have broken something wrt prepare(). Will have a look tomorrow.

[ankoh]

I'd prefer null over undefined here as return type for nulls.
Maybe res === undefined || res === null ?

[hannes]

So what happens if we get undefined then?

[ankoh]

maybe out_validity[i] = (res == undefined || res === null)? 0 : 1; ?
I just thought that supporting an actual null as return value might be semantically better.

[hannes]

The Node 14 crash seems related to node-ffi-napi/ref-napi#54

[Mytherin]

Should these files not be ignored by the format script already?

[Mytherin]

Is there a better way of getting these include directories?