Improve performance of eventsource parser

[gr2m]

Description

This is a follow up to #5325 (comment).

I created a simple benchmark that compares @rexxars eventsource-parser and @ai-sdk/provider-utils' own implementation based on the bug report by @wong2:

https://github.com/gr2m/eventsource-parser-benchmark/#readme

Based on this particular use case, eventsource-parser-3.0.1 is

1. ~100x faster than eventsource-parser-3.0.0, and
2. ~10x faster than @ai-sdk/provider-utils' own implementation

I couldn't yet figure out where the performance difference between eventsource-parser-3.0.1 and @ai-sdk/provider-utils' own implementation is coming from. I'll look more into it this coming week

[lgrammel]

switched back to eventsource-parser