StringReader constructor

A Reader implementation that reads characters from a String buffer.

This reader allows sequential, asynchronous reading of characters or character blocks from an in-memory string.

Common use case:

final reader = StringReader('hello');
final buffer = List<int>.filled(5, 0);
final read = await reader.read(buffer); // buffer now contains [104, 101, 108, 108, 111]
await reader.close();

The reader supports mark/reset operations for backtracking.