`\r` and `\n` arriving in separate chunks breaks line separator detection

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* Operating System: Any
* Node Version: Any
* NPM Version: Any
* csv-parser Version: 3.0.0

### Expected Behavior

The assert in the repro passes.

### Actual Behavior

The assert in the repro fails.

### How Do We Reproduce?

```ts
import { deepStrictEqual } from 'node:assert';
import { Readable } from 'node:stream';

import csvParserFactory from 'csv-parser';

deepStrictEqual(
await Readable.from(["foo\r", "\nbar\r\n", "baz\r\n"])
.pipe(csvParserFactory())
.toArray(),
await Readable.from(["foo\r\n", "bar\r\n", "baz\r\n"])
.pipe(csvParserFactory())
.toArray(),
);
```

### Cause

This happens because `csv-parser` is fine with `nextChr` being `null` when `this.state.first` is `true`. Instead it should wait until there's at least one more byte or the input stream is closed.

Start by running the provided Readable-stream repro against csv-parser and inspect the parser's handling of nextChr and state.first when chunks split a line separator. Done means the chunked input produces the same rows as input containing complete \r\n sequences, with the existing assertion passing.

Written by the indexing model from the issue text.