Potetial to improve perf

[pi0]

Currently we use a dynamic array as the buffer for splitter core (https://github.com/unjs/scule/blob/main/src/index.ts#L30). Since we know about the (maximum) string length, we could instead make an array with known length and assign chars too.

Howevewr this needs some benchmark script to verify, just made issue to not forget! (PR welcome for bench > try)