Allow vgplot to use DuckDBClient

[angrytongan]

From https://talk.observablehq.com/t/observable-framework-using-duckdbclient-sql-with-vgplot/9676/3:

I would like to load data using DuckDBClient.sql() and display with vgplot. I am able to do this with Observable Plot:

```js
const sql = DuckDBClient.sql({ rand: FileAttachment("./rand-xy.csv") });
```

```sql id=data
SELECT * FROM rand
```

```js
display(
  Plot.plot({
    height: 200,
    marks: [
      Plot.dot(data, {
        x: "x",
        y: "y",
      }),
    ],
  })
);
```

but not with vgplot:

```js
const sql = DuckDBClient.sql({ rand: FileAttachment("./rand-xy.csv") });
```

```js
display(
  vg.plot(
    vg.height(200),
    vg.dot(vg.from("rand"), {
      x: "x",
      y: "y",
    })
  )
);
```

@mbostock responded:

Overriding the definition of sql doesn’t work (currently) with our implementation of vgplot. You’ll need to use the SQL front matter instead.