feat: add Decimal type

[universalmind303]

No description provided.

[Bidek56]

@universalmind303 Do we need to add documentation for this feature?
Should we add this test?

  test("writeParquet", (done) => {
    const df = pl.DataFrame([
      pl.Series("doo", [1, 2, 3], pl.Decimal),
      pl.Series("foo", [1, 2, 3], pl.UInt32),
      pl.Series("bar", ["a", "b", "c"]),
    ]);
    const pqFile = "./test.parquet";
    df.writeParquet(pqFile);
    const newDF = pl.readParquet(pqFile);
    expect(newDF).toFrameEqual(df);
    fs.rmSync(pqFile);
    done();
  });