Reader is slow and synchronous read-string should be supported

The current reader is slow.
One reason is that it uses subs (which performs in O(n) in Dart) thus turning linear processing in quadratic ones.

The current reader should be able to act bot in sync and asynchronous manner, so that read-string works.

IMO we can't be content with batching input until complete and then synchronously parsing it (like the JSON codec does). The main reason is the REPL. Otherwise you have to have some pre processing to detect top-level ends. The other reason is being able to read large files.

Plugging into dart:convert would allow better integration with dart libs.
I believe we can represent chunked conversions inner state using something inspired by transducers https://api.dart.dev/be/175791/dart-convert/Converter/startChunkedConversion.html

Start by locating the reader implementation and its use of Dart subs; then review dart:convert's Converter/startChunkedConversion model. Determine how synchronous and asynchronous read-string support should work for the REPL and large-file input, while avoiding the reported quadratic processing. Done means the reader supports both modes and integrates with Dart conversion APIs.