To match the python feature: Unstructured-IO/unstructured-python-client#125

Add a client-side param called `splitPdfPageRange` which takes a list of two integers, `[start, end]`. If `splitPdfPage` is `true` and a range is set, slice the doc from `start` up to and including `end`. Only this page range will be sent to the API. The subset of pages is still split up as needed. If `[start, end]` is out of bounds, throw an error to the user.

To match the python feature:
Unstructured-IO/unstructured-python-client#125

# New parameter
Add a client-side param called `splitPdfPageRange` which takes a list of
two integers, `[start, end]`. If `splitPdfPage` is `true` and a range is
set, slice the doc from `start` up to and including `end`. Only this
page range will be sent to the API. The subset of pages is still split
up as needed. If `[start, end]` is out of bounds, throw an error to the
user.

# Testing
Check out this branch and set up a request to your local API:

```
const client = new UnstructuredClient({
    serverURL: "http://localhost:8000",
    security: {
        apiKeyAuth: key,
    },
});

const filename = "layout-parser-paper.pdf";
const data = fs.readFileSync(filename);

client.general.partition({
    partitionParameters: {
        files: {
            content: data,
            fileName: filename,
        },
        strategy: Strategy.Fast,
        splitPdfPage: true,
        splitPdfPageRange: [4, 8],
    }
}).then((res: PartitionResponse) => {
    if (res.statusCode == 200) {
        console.log(res.elements);
    }
}).catch((e) => {
    if (e.statusCode) {
      console.log(e.statusCode);
      console.log(e.body);
    } else {
      console.log(e);
    }
});
```

Test out various page ranges and confirm that the returned elements are
within the range. Invalid ranges should throw a useful Error (pages are
out of bounds, or end_page < start_page).