feat: add output_schema method to queries (#2717)

This is a helper utility I need for some of my data loader work. It
makes it easy to see the output schema even when a `select` has been
applied.

docs/src/js/classes/QueryBase.md

@@ -140,6 +140,25 @@ const plan = await table.query().nearestTo([0.5, 0.2]).explainPlan();
 
 ***
 
+### outputSchema()
+
+```ts
+outputSchema(): Promise<Schema<any>>
+```
+
+Returns the schema of the output that will be returned by this query.
+
+This can be used to inspect the types and names of the columns that will be
+returned by the query before executing it.
+
+#### Returns
+
+`Promise`&lt;`Schema`&lt;`any`&gt;&gt;
+
+An Arrow Schema describing the output columns.
+
+***
+
 ### select()
 
 ```ts