feat: add pcre2 as optional feature

[wheynelau]

Motivation: Exploring performance profiling and noticed onig showing up in the profiles and tried swapping for pcre2. Happy to get some feedback - I'm not deeply familiar with the tradeoffs.

I have validated that all tests pass and the benchmarks shows that its better for GPT2 and Llama3 models:

Benchmark	main (onig)	pcre2
bpe-encode/BPE GPT2 encode	1705.7±19.00ms 3.6 MB/sec	1422.8±10.89ms 4.3 MB/sec
llama3-encode/llama3-encode	1912.2±21.94ms 3.2 MB/sec	1601.6±5.81ms 3.9 MB/sec
bpe-encode/BPE GPT2 encode, no cache	2.5±0.04s 2.5 MB/sec	2.1±0.02s 2.9 MB/sec
llama3-encode/llama3-offsets	257.9±7.03ms 24.0 MB/sec	240.8±3.05ms 25.7 MB/sec
llama3-encode/llama3-batch	340.9±5.99ms 18.2 MB/sec	319.3±3.05ms 19.4 MB/sec

Commands used:

cargo bench --no-default-features --features onig,progressbar,esaxx_fast -- --save-baseline main
cargo bench --no-default-features --features pcre2,progressbar,esaxx_fast -- --save-baseline pcre2

Based on perf these were my CPU samples:

func	onig	pcre2
NormalizedString::split	5.58%	1.44%