Add nth fn binding (#212)

The [nth fn](https://docs.rs/polars/latest/polars/prelude/fn.nth.html)
in rust polar is different from what is available in the [python
repo](https://docs.pola.rs/py-polars/html/reference/expressions/api/polars.nth.html#polars-nth).
Not sure what's the convention for this nodejs library, but just
exposing what the rust library does is easier and satisfies our needs.

__tests__/dataframe.test.ts

@@ -691,6 +691,28 @@ describe("dataframe", () => {
     });
     expect(actual.nChunks()).toEqual(1);
   });
+  test("nth", () => {
+    const actual = pl
+      .DataFrame({
+        foo: [1, 2, null],
+        bar: [6, 2, 8],
+        apple: [6, 2, 8],
+        pizza: [null, null, 8],
+      })
+      .select(pl.nth(2));
+    expect(actual.columns).toEqual(["apple"]);
+  });
+  test("nth:multiple", () => {
+    const actual = pl
+      .DataFrame({
+        foo: [1, 2, null],
+        bar: [6, 2, 8],
+        apple: [6, 2, 8],
+        pizza: [null, null, 8],
+      })
+      .select(pl.nth(2), pl.nth(3));
+    expect(actual.columns).toEqual(["apple", "pizza"]);
+  });
   test("nullCount", () => {
     const actual = pl
       .DataFrame({

polars/index.ts

@@ -53,6 +53,7 @@ export namespace pl {
 
   // lazy
   export import col = lazy.col;
+  export import nth = lazy.nth;
   export import cols = lazy.cols;
   export import lit = lazy.lit;
   export import intRange = lazy.intRange;

polars/lazy/functions.ts

@@ -117,6 +117,34 @@ export function cols(...cols): Expr {
   return col(cols.flat());
 }
 
+/**
+ * Select nth column index in a DataFrame.
+ * @param n - Column index to select, starting at 0.
+ * @example
+ * ```
+ * > df = pl.DataFrame({
+ * > "ham": [1, 2, 3],
+ * > "hamburger": [11, 22, 33],
+ * > "foo": [3, 2, 1]})
+ * > df.select(nth(2))
+ * shape: (3, 1)
+ * ╭─────╮
+ * │ foo │
+ * │ --- │
+ * │ i64 │
+ * ╞═════╡
+ * │ 3   │
+ * ├╌╌╌╌╌┤
+ * │ 2   │
+ * ├╌╌╌╌╌┤
+ * │ 1   │
+ * ╰─────╯
+ * ```
+ */
+export function nth(n: number): Expr {
+  return _Expr(pli.nth(n));
+}
+
 export function lit(value: any): Expr {
   if (Array.isArray(value)) {
     value = Series(value);

src/lazy/dsl.rs

@@ -1625,6 +1625,11 @@ pub fn last() -> JsExpr {
     dsl::last().into()
 }
 
+#[napi(catch_unwind)]
+pub fn nth(n: i64) -> JsExpr {
+    Expr::Nth(n).into()
+}
+
 #[napi(catch_unwind)]
 pub fn cols(names: Vec<String>) -> JsExpr {
     dsl::cols(names).into()